Hey, its good you care so much about her, but you cant make her care

She sounds like now is just not the time for her to change anything, and if she doesnt want to then she doesnt have to. Ultimately, our motivation has to come from within. If I was you, I would simply say to her something like "hey, I am losing weight for my health and I am worried about how your health is being affected by your weight too, so if you want to then I would be happy to be your diet/weight loss buddy" and leave it at that. It really is something she has to come into of her own accord