The question is, does he need it? If he's running XP, I'm betting it's an older computer. If it's an older computer, does it have more than 4GB RAM? If yes, then he would benefit from being able to go more than 4GB. If he doesn't have more than 4GB, then he won't benefit from going 64bit.
Also, make sure that his computer meet the minimum specs for Win 10 BEFORE you start. Also, what kind of computer (what are the specs)?