Wow, I just went to a training on this with my company. The topic was having difficult conversations, and as family advocates, my co-workers have the hardest time addressing issues of...hygeine! The goal here is to have the person understand that you are concerned for THEM. Say privately , " I have something to talk with you about that may make you feel ...(pick a word that fits) uncomfortable, angry, defensive, etc. But I've noticed that you have a hard time (pick one) making friends, getting help, finding work etc. I think that it may be your personal body odor that is making others uncomforatble, and I just wanted to let you know in case you were unaware."
Now the trick here is to make sure that you don't lose your credibility here. They will be defensive, and will probably make verbal attacks. Stay calm and even professional. Its not about you. Say, "This is what I am talking about right now. We can discuss this other stuff later."
What a difficult topic to have to address. Good luck!
Department of Mental Health training, Mass.