Question: What is the best way to tell someone that they need to use deodorant?
Options:
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.
It's the one thing we probably hate doing the most. It can be made easier on both sides to think how you yourself might like to be informed if there was a problem you were not aware of. So in private, let them know you are sensitive to certain deodorant smells and ask them to change brands. Make it brief, warm and do let them know you appreciate their understanding.
Answer: 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.