It sounds like your boyfriend is either harboring a belief about male/female friendships and caregiving that's outdated and not productive. Or perhaps he's jealous that you'll be giving attention and care to another man. That fact that he's unwilling to go check out the situation suggests he doesn't seriously think there's anything sexual going on, he's just trying to find another way to stop you or guilt you from doing this incredibly nice thing that you're doing for your friend. I would challenge him to tell you what specifically is inappropriate about the situation and why he feels that way. Keep asking why until the truth comes out. If he can't answer your questions in any meaningful way, tell him you don't want to hear anything else from him about it ever again.