I was reading the question and answers at Can someone decide to be hit?, and the answers spoke of using the rule of Advantage/Disavantage. Which got me thinking...
We had a situation a few weeks back where a player was roleplaying a jerk and was intentionally missing their shots to annoy another player/character. (Funny scene.)
When rolling with Advantage, are you required to use the higher of the two dice?
When you have either advantage or disadvantage, you roll a second d20 when you make the roll. Use the higher of the two rolls if you have advantage. (PHB p.7)
There's no "you may" or "you can" in there, you just do.
(Note that some sources of advantage are optional: if you choose not to "accept" advantage then you, of course, wouldn't roll two and take the higher. And it's not hard to give oneself disadvantage, opposing your advantage. But your question said that you have advantage, in which case there's no option on the dice.)
(Note also that the Lucky feat interacts with (dis)advantage in a counterintuitive way according to Sage Advice.)
One thing I always loved about D&D is, that it's rules are meant to be more of an inspiration other than something that needs to be held on with all force. If you play a card game and someone lies down a card, where the players are not sure if that is a legit move or not, you are very likely to check the rules for it and correct whatever the player wanted to do. But D&D, but especially D&D5 is made extremely openly, so you can play the game the way you want to play.
So in this case. If your partner wants to shoot but not hit it's target, why shouldn't he be possible to aim more to the right. Even though it is a fantasy world, it is still a world with working physics. So why should the arrow fly a curve and hit your opponents head just because you had to roll on advantage and unintentionaly rolled a 20. Same counts for any ability check/saving throw. Even though I can't find it right now, I always had in my head, that the rulebook said something like "...but you can always decide to fail the check without rolling a die". But here are some quotes from the preface on page 4 of the player's handbook that show exactly what I mean in my first paragraph:
To play D&D, and to play it well, you don’t need to read all the rules, memorize every detail of the game, or master the fine art of rolling funny looking dice. None of those things have any bearing on what’s best about the game.
[...]
The second thing you need is a lively imagination or, more importantly, the willingness to use whatever imagination you have.
[...]
Above all else, D&D is yours. The friendships you make around the table will be unique to you. The adventures you embark on, the characters you create, the memories you make—these w ill be yours. D&D is your personal corner of the universe, a place where you have free reign to do as you wish.
but especially:
Go forth now. Read the rules o f the game and the story of its worlds, but always remember that you are the one who brings them to life. They are nothing without the spark of life that you give them.
So if anything you do or want to do makes the situation funnier or more exciting just do it. If you have a good DM, he will allow it, even though it would break many rules. As long as it has no negative effects on the other players (not neccessarily characters, since some players are also happy with that) why don't give it a try? Even Chris Perkins himself makes many of these descisions (see their shows at PAX for instance) because they give everyone playing and watching a much better feeling about the story, its characters, but especially its players.
My experience with role-playing pen and papers has always been, that its rulebook(s) should never be seen as a book of law, but rather as a framework that does nothing more, than to give your game structure.
EDIT: Ah, sorry. But other than that the rules would say that you take the higher value as nitsua60 already mentioned, yes. But as I said. It should never be something you need to stick to, because it is written down in some book. At least not in dnd
