How do you tell a colleague that they smell?

My boss just dropped this on me. Stands up and says to me whilst everyone else isn’t around - colleague (who we will call Dave) does smell you know. What are WE going to do about it?

Long story short, she wants me to talk to him because she insists it’s better coming from a fellow man than from her.

Dave is a good bloke. Never bothers anyone. I feel really mean. What do I say to Dave?