I feel like there is a way. but it's complicated i guess.
You plug your phone in to his computer with the App, drag it to where it says either Apps or your iPhone. Then, since it's his, I believe he will need to "verify" this, by logging in to his Apple ID on his computer.
The thing I don't know is, if it will wipe out your data because its a different computer or different Apple ID or something.
or or or
try signing in to his Apple ID on your iPhone through your settings and then go to the game you want that he has and maaaaybe just maybe it shows that he has it and you can just download it at no cost? maybeee?
I'm just throwing ideas out.
:s don't punch me, but I did warn you if you lose your data, so make sure you read everything that possibly pops up, but if you don't trust it, i wouldn't do it.