I don't know if there is a good way to approach that topic with a woman.
If it were a guy you could say, "Hey fuckface, your fucking teeth are nasty and about to fall out of your head. Go to a dentist, get some pliers or stop fucking smiling, 'cuz, really, it's fucking nasty to look at that black shit clinging to you teeth like a baby monkey to it's mother's teat."
Of course with a woman you have to be a bit more tactful. I don't even have a suggestion. Well maybe I do...How about, "You have such a pretty smile. Too bad it's being ruined by your poor dental hygiene."
