estrogen during puberty will cause her to gain fat weight, to grow breast, and to store more fat in her hips and thighs. this is very common during puberty as her estrogen levels rise
but she will continue to grow and develop
and this is not her final body shape
you should not put so much pressure on her to change her body shape
her biology is something she can not control.
her body's puberty and estrogen level is something she can not control.
she may not grow into a thin swimsuit model no matter how much she diets or exercise.
this is not because she has a medical problem
taking metformin most likely is not the right thing for her.
she most likely doesn't have pcos.
metformin can have side effects. and can cause upset stomach and gastritis.