I would prob make a joke about it instead of being serious and saying "dude.....you smell". Like if my bf's breath smells I'll just plug my nose when he goes to kiss me and ask him if he has been helping the cats clean themselves haha.

I guess it depends on how close you are to this person? If a close friend told me I smelled I would prob take it as a concern for me instead of an insult. But it depends on their personality too. If you think she would be offended then maybe find a new way to say it or hint? Like "hey, what kind of deodorant do you wear?" lol I dont know cause I'm so blunt I just tell my friends they stink.