Onion Armor is an armor obtained through the Crystal Tower I believe. Feel free to correct me if I'm wrong, as I haven't gotten that far yet. So unless you have access to the Crystal Tower, I don't think there's gonna be anyway you're gonna be able to help your friend.

Farming in any game takes a while to do, whether you're lucky or not. I don't think he should just quit because he can't seem to get it. Just tell him to farm to his patience limit, take some time off, take a breather, build up his patience and come back and try again.

If his level of desire for this armor is high enough, this much is achievable.