Heh, before you have this talk with her, just be cautious. Girls are so sensitive about their weight, it really isn't logical, so seriously... tread with caution.
Now with that said, I'll voice my opinion, but first I'd like to know, how tall is she? 135 lb is a perfectly respectable weight for a women, even a short one. Now, that's my opinion, but that's not what drives her to lose weight, is it?
No, the reason she wants to lose weight is either because she feels like others aren't treating her correctly because of it, which I doubt but is possible, or she doesn't feel attractive enough for you.
The first thing you should do is just ask her why she cares so much about losing weight when you find her extremely attractive already. Assuring her that you love her like she is and really don't want her to lose any weight is a solid starting point. :)