Can you just photoshop her back to skinny??
On a more serious note umm first think about what has changed in her life that has made her gain weight? Did she start eating bad? Stop working out? Just leave high school, off to college etc...
People tend to gain weight/eat due to lifestyle change or as comfort for some area in their life that has changed. Depression, anxiety, job, stress etc...
Think about what changed that lead to her weight gain and then if it's something you can help her deal with talk to her about the issue (stress, job, money etc..) Don't try to fix her, you will likely only make it worse, try to listen to her problem that is causing her to eat, gain weight. I'm betting something about her or your relationship has changed along with her weight gain. Find that problem and you will find a solution to the other.