These days I'd be grossed out by myself if I had the same lack of hygiene schedule. I am pretty sure that a deficient hygiene regiment is at least in part responsible for my lack of social success in high school and before. I think a big part of the problem is a lack of self-awareness. If you could get this person to experience good hygiene practices, she may see how gross her former habits were and not go back.
Another issue is that the encouragement of good hygiene should be presented as a positive thing, not as something that she experiences resentment for because other people don't accept her. I used to resent other peoples' apparent lack of depth beyond clothes and hair, and this was a major impediment for me to connect with them. In retrospect, maybe I still wouldn't have connected with them, but at least I could have enjoyed the better feeling of being clean and neat.
Oh, and about the teeth. If you must choose an issue upon which to focus attention, it should be the teeth. That's the one part of the body that is damaged if you don't keep it clean! Make sure she flosses, if you can.
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A boy and his dog can go walking
A boy and his dog sometimes talk to each other
A boy and a dog can be happy sitting down in the woods on a log
But a dog knows his boy can go wrong