You might need to reconsider a future with her.

If as you said, she gets very thin and you aren't attracted to her--then what? If your feelings for her would remain regardless of what she looks like, then the answer is easy. Let her do as she wishes as it will make her more confident.

If the answer is no, then your relationship will only bring the both of you grief.

Oh hell yes! I see this at work with some of the ladies. And it takes a Herculean effort on my part to pretend that I don't notice