It must be frustrating to see a problem and have little ability to have impact. This situation is far too complex for a simple answer. You didn’t indicate what kind of relationship you have with your daughter or with the grandparents who have been raising her. You also didn’t share how much contact you have with them. Although I appreciate and share your concern, my guess is that your ability to intervene is minimal.
That being said: Body image and weight issues are common among teens. Shaming doesn’t help. Threats and bribes don’t help. A supportive conversation with your daughter that lets her share her own concerns might. It’s likely she is anxious and discouraged. In that case, a conversation about how you might help her feel better about herself and more empowered to take care of herself will be more productive than insisting on a diet plan.
If the family is receptive, you may be able to share that information. If not, all you can do is calmly share your concerns and offer to help when people are ready to accept it. Do remember that doing what you can to preserve your relationships with everyone is more important than insisting that they resolve this particular problem. With a stronger relationship, you will have more influence.
I wish you well.