You could always offer to walk with her and just make her go a little faster to keep up with you everyday. Once she really starts moving you'll get her to see how much better she'll feel. I actually forced most friends who wanted to hang out with me to walk where ever we went since I never got my license or a car till I graduated and I wanted to be anywhere but home.  I wouldn't push eating salads too hard as that could make her not like the fact she has to "give" up something she enjoys, as I maintain low weight by just eating smaller portions of what I like. I am also an incredibly picky eater, so if she's similar maybe just talk to her about portion sizes.  In high school most of my friends were fat, I tried the same, but it never worked for them. I wish you luck in helping her become human.