How do you do this: What is the best way to tell someone that they need to use deodorant?
It's one of those things we would rather avoid. However, the best approach is put yourself in their shoes. Wouldn't you want to know if it were you that had a body odor problem? Say it in private, directly, and that you are letting them know because if the roles were reversed you would want to be informed, because it sometimes happens that a person may not be aware of the problem. Make it brief, warm, and thank them for letting you speak to them about it.