Ok so my friend smells of B.O. SO BAD, and I have to tell him. Any ideas on how to tell him nicely?

Yikes, this is a tough one.

I would sit him down and say;

"You're my friend, I care about you, that's why I have to tell you this because I don't want someone to say something hurtful if I can prevent it. You have bad B.O."

Then offer to help him find some good soap and underarm deodorant. Try to make it fun when you go.

He'll probably be more embarrassed then hurt, but better embarrassed with one person that cares about him then a whole bunch of people that may just tease him.