ultimately, you gotta be real with her. Sounds like you already are, by saying, hey look, i think you are mad sexy the way you are.

then though you have to put a foot down. like: tell her she can pick one:

Option 1 Continue doing what she is doing, and not lose weight (which is FINE!!) , but then she must accept herself and STOP COMPLAINING ABOUT IT.

or

Option 2 Get in the gym/go for a run and eat better. Tell her you will be supportive of her, always emphasizing that these changes are FOR HER!! (This is not a lie, its the truth).

Obviously the tricky part is: how do you deliver this message. You can't do it in such a straighforward way as I have laid it out above. That part, is the part that i tend to not be as good at, so I don't feel qualified to be giving advice on. Anyone else who wants to add to this, please do