I have to agree with the others - kids shouldn't be worried about their weight and putting a diet plan in another young, untrained person's hands could be quite dangerous. If it's that big of a deal, she needs to talk to her parents and doctor.
Why do you think she doesn't have a scale at her house?
I know you want to help, that's great and I admire that you want your friend to be happy and healthy, but you need to encourage her in a different way. You can't take into your own hands a diet for your friend because you don't have the crucial knowledge needed to diet and lose weight safely.
If you really want to help your friend you need to step back from controlling some sort of diet for her and have her talk to her parents about eating healthier and exercising more.