I am sorry this is happening to your daughter. Have you taken her to the doctor to take a look at her skin? Here are images of different skin conditions related to obesity. I would have her wear comfortable loose clothing...nothing binding or scratchy. You might put a protective layer of vaseline on her skin to prevent friction. And keep her cool so she doesn't sweat so much...that will only aggravate the problem.
Also here is our Diet and Exercise site to help with your daughter's weight...losing the weight is one sure way to help with her skin condition.
Hope this helps...please let us know if you need any further resources.