I would engage the person in conversation and then talk about MYSELF needing to get to the dentist, and maybe while I am at it, ask the dentist about tooth whitening procedures, because I feel my teeth could benefit from a treatment. If the person would answer back that my teeth look fine, I would say, thanks, but this is something that has been bothering me, and I think I should go for a professional opinion, because the smile is one of the first thinghs people notice.
I learned this idea from a Rov, who whenever he needs to give mussar, tries to give it in a way that states that so many of us, himself included, are guilty of whatever it is. He gave a great mussar shmooze on L”H and on hakoras hatov and started out by telling a story (ostensibly about himself) where he felt he possibly had been nichshal, and needed to improve himself. The people listening were able to be mekabeil the mussar, because we all had a sense of “we’re in this together.”
If your friend can infer from what you are saying that she, too, could benefit from this procedure (and then she actually follows through)without you actually telling her her teeth look awful, then you will have done her a chessed.