First of all, it's not polite to take someone's phone and see what someone has in it (at least not here where I live), and secondly, why would your "friend" have a BTC wallet on his smartphone if he doesn't know how to use it?
Finally, someone is having the same thought as mine. It baffles me how everyone is ignoring the part Op said he checked his friend phone and focus on adding to what he puts up there. Checking someone's phone is invasion of privacy that we should frown at. You don't just check people's phone to the extent of going to their wallet. What exactly are you looking for? It's not peculiar to where you live alone, it's a general ethic everywhere at least, for sane people.
Maybe his friend is keeping the BTC wallet for fancy purpose or probably got the phone from someone who forgot to wipe out his wallet. That would be stupid of both of them thou. Some people choke their phone storage with unknown apps that they neither have no use of it, I'm not too surprised.