There are a few options.
I would say that the best option is to simply take away the magic items. I know you said that the DM doesn't want to take away the items, but as a DM, he must to learn to say "No" at some point, and this will likely be the healthiest option for the game. Whether it's a Poof! they vanish before the parties very eyes, or they get stolen in the night by a thief, they should be taken away. Those items are far to powerful for players of that level to be using. I personally recommend using the "rumors have spread of the players being in possession of incredibly powerful magic items" and, obviously, someone else is going to want those items. Getting them back isn't out of the question, however, as while they're working on the main quest line, they could pick up hints and rumors about weapons new owners(s). If he's really, truly dead-set on letting the players keep their items, then one more option is to have the magic in the items start to weaken over time, and make it a bit of a side quest that the players need to have them
- Option 2: The enemies are evolving
The next option is to make everything more difficult to kill, without making them more likely to kill the players. Give creatures additional HP, and stack on resistances. I recommend some minor fiends, perhaps a squad of bearded devils, with bonus HP as they can resist most of that elemental damage, but not the base weapons damage. Alternatively, bring in an rival party, who are sort of reverses of the players, and instead of having weapons that are incredibly OP, they have armor that make it incredibly difficult for the PCs to harm them. Of course, if the PCs get close to killing them, they flee (to prevent the players from getting additional OP items). This one I don't particularly recommend, since you said that the DM is rather inexperienced.
- Option 3: Pits and darts and boulders, oh my!
Non-combat encounters are likely to be more challenging, since the players only have enhanced attack power, not problem solving power. Lets say they're raiding an abandoned temple. The temple is full of traps, like spike pits, swinging blades, and flood rooms, and puzzles but there are basically no creatures that are worth fighting, maybe a giant rat or two, something that's not big enough to set off pressure plates, and nimble enough to climb over/out of pits and the like. Obviously, their weapons aren't going to do them much good against things that can't be fought.
- Option 4: I roll to seduce the armchair
The other form on non-combat encounter: the dreaded... Social Challenge! *there is a crack of thunder, gasps are heard, the crowd murmurs, a soft scream is heard as one woman faints, a Dun, dun, DUNNNN can be heard in the distance*. Yes, you heard right, your +2 Sword of Exterminating will grant you no power here! Perhaps the players need the aid of one of the local lords, or perhaps they're trying to get something out of an informant. Either way, their weapons aren't going to help them out much, because they need the person that they're trying to influence alive and friendly in order to get to the next step of the adventure. The utilization of Skill Challenged is likely to be useful for this option.
Last, and probably most importantly, both of those homebrew weapons are busted above and beyond the other magic weapons. The sword should be nerfed down to 2d6 (a regular greatsword) damage, plus 1d4 acid that lasts indefinitely but does not stack and allows a DC14 Con save at the end of the creatures turn to end. The staff should only deal 1d8 + 1d6 Radiant. This, and I can't stress this enough, needs to be done regardless of what other changes (if any) the DM makes. These two weapons are simply to powerful to be allowed in the game, especially that sword. Also, I assume that with the "rocket launcher" is has a 120' range not 120' radius. I also assume it runs on charges, because if it doesn't that needs to be adjusted as well, to something along the lines of 7/day, with 1d6+1 back every morning, and if the last charge is used, roll 1d20 and if you get a 1 it crumbles to dust, as per usual for magic wand equivalents.