Removal of players' agency.
While there's something to what others say about same-paging, this seems like a more serious series of GM failures, most of which are pretty much never OK regardless of same-page agreements.
Ignored player intent
The players explicitly stated they were stepping outside to discuss something privately. The GM had no apparent confusion about this: he understood that the clear intent of the players was to move their PCs to where they could safely have a private discussion.
The GM subverted the player's agency to do so, by allowing a roll to make their location choice invalid to their privacy, and to have their whispers audible through a solid wall. He didn't even give them any kind of saving throw or ability to detect or act upon the eavesdropping, nor a stealth check to succeed at being stealthy.
"Is that all?"
This question doesn't absolve a GM from giving players the info they need to make their decisions.
Asking "is that all" when players have their PCs move outside to speak privately where the other PCs can't hear them, then only afterwards rolling to see if they're somehow still within whisper-hearing-range of another PC, is like asking "is that all?" after they declare their actions to sneak up and silently take out a goblin, and only then rolling to see if the goblin's riding a Dragon.
I'd go so far as to say: never ask this, if what you mean is "There's something you missed, can you guess what it is?". Because the challenge of the game shouldn't ever be "can we second-guess the GM?" Screwing the PCs is not the job of the GM.
"are you sure you want to...?"
Whenever they might not have deliberately chosen to use their agency to attempt the risky and/or dumb thing they just said they were, you can and should use this question.
"Are you sure you want to step out of the shadows and offer a sandwich to the necromancer, surrounded by three dozen armed skeletons, literally crackling with magical energy... while you're wet and naked?" "Yes, I'm sure." "Oookay. Charisma roll. Disadvantage..."
In this case, the unasked question was "Are you sure you want to stand by the toiletpaper-thin wall, as you have this discussion in Common?".
Or don't even ask: assume common sense
Sometimes, clarifying questions aren't worth asking. For example, the GM here wisely didn't ask "are you moving somewhere that there's no chance of being overheard and still talking in Orcish, or standing somewhere that even a sorta whisper has a 1-in-8 chance of being overheard and talking in common?" ... because, if he had, he knows what they'd have answered. They already stated their intent: to speak privately, not "sorta privately, assuming everyone stays fast asleep", because in that case they would have just stayed in the room with all the sleeping people.
Assume that PCs trying to avoid being overheard, won't talk loudly enough to be overheard.
Assume that in any conversation between two people who share a common language, they will use the least-common language shared by all present.
Assume that people take a long rest have a rotated watch.
Assume that people going on a long trip will know whether they need to buy rations.
Assume that you aren't playing a game of "gotcha" against your players. Screwing the players or PCs is not the job of the GM. Screwing the PCs is the job of the NPCs and situations they encounter.
Good rolls don't grant superpowers
The urge to "reward" good rolls is strong. And if someone gets a good roll with disadvantage, it feels like they got an even better roll, so need an even better reward than if they had got the same result without disadvantage. Treating an 18 and a 19 as if it were higher than both.
But statistically, there's negligible difference between getting an 18 on a D20 (15% odds), and a 6 on a d6 (16.7% odds), so even if you felt the need to reward good rolls... it's not a great roll. And "at disadvantage" should never mean "automatically get a better result if you roll well". Quite the opposite, if anything.
So at those odds, the best the player should have expected was the ability to detect that someone was speaking.
Even a 20 (5% chance) should only give a little extra information: perhaps the muffled speech patterns could let them identify how many people was talking, or who was talking, or what tone their speech was taking.
Hearing precisely WHAT they were saying should not have been possible from that roll in the first place.
PvP, if allowed, should be a two-way street
If the person making the roll then wants to actually eavesdrop well enough to hear words, he needs to take actions, make rolls, etc. He has to do a thing: eavesdrop. That's not a passive action, that's an attack on the agency of the other players. He can't just get it for free.
So, as their eavesdropping action, he might decide to open a window to hear better. Then the speakers will get to notice and respond to it. "A window opens nearby" -> "we switch to Orcish".
Or he could sneak out to listen. In which case they'd have to make a stealth check, and the speakers should get a perception check, but they fail, you're in trouble as a GM.
You can't force players to screw their PCs
By putting the players in the untenable position of "now say the stuff that you don't want to have overheard, for this player to overhear"... you have screwed your players, and yourself.
You're asking them use their imagination to screw themselves. Screwing the PCs is not the job of their players.
HOW DO YOU RECOVER?
Failure shouldn't be a blame game. Everyone will fail. GM slipped up, that's a good thing, means he had a chance to learn things. You don't get experience by succeeding all the time! So how can it be avoided in future? And how can this be fixed once it inevitably happens again?
So, OK: the GM retconned the private talk into a non-private one. Or maybe they failed a perception check to notice someone sneaking up to eavesdrop.
Either way, the GM's got himself into a position where he's violating the rule of not forcing people to screw themselves. Since he can't force someone to screw their own character, it falls to him as a GM to storytell his way out of the corner he's painted everyone into.
This corner-avoidance is a core skill of GMing. Ideally it should be invisible to the players.
For this recipe, you will need one reward, and one ass-pull.
A reward
He should give the eavesdropper something - but that something shouldn't give away the other players' whole farm for the cost of a single passed skill check, nor should it require the players to screw themselves.
The GM can tell the spy that they saw the two characters talking together. The tone of their voices was urgent and conspiratorial. And if he knows what they would have been talking about, he can give additional clues. Perhaps one pointed at the clocktower, and passed some gold; the other checked his crossbow. The spy might incorrectly assume one's paying the other to snipe from the clocktower, rather than saying "be ready for a fight in 45 minutes, go buy some more bolts if you need them" ...and that's OK.
An ass-pull
But after giving that minor reward for showing the initiative to snoop, the GM then has to pull a rabbit out of the hat to save face for the players being snooped on, and avoid them having to actively screw themselves. So roll the dice for color, look on the "town encounters" table, and there's an encounter.
An interruption, a nightwatchman walking down the road. A window flying open and the contents of a bedpan splashing right beside them as a woman screeches "you think I can't hear you whispering outside my room? Whatever you're plotting, do it somewhere else, you 'orrible little thieves!" A footpad attacking the spy for lurking in his territory. A cat chases a rat past them. A snake-oil salesman rolls up in his cart and decides to try to try making one last sale before driving his cart out of the town in the dead of night for no particular reason. A street urchin who's also been spying on them sends a shingle sliding off the roof with a clatter. An ornate cart rides through, clearly in a hurry. It starts to rain. There's a noise from the stables. The innkeeper sticks his head out, making sure they're not running off without closing out their tab...
Literally anything that gives them the narrative excuse to pivot so they're no longer screwing themselves, so that they don't have to come up some silly retcon themselves.