I have been on prosac twice and didn't have issues with weight gain either time. That is not to say it isn't possible it just wasn't an issue for me. I would be more concerned if your daughter was "overweight" with the extra weight gain if she is then maybe okay there is reason for disliking the weight gain but if she is within a normal weight for her height I can understand her not liking it but it may be healthier for her. She can always workout and try to tone up what she has too, if losing the weight is not an option.
Have any of her other medications changed? If so then maybe it is something else she is on maybe a new enzyme? When I had to change enzymes to Pancreaze from Ultrase I gained a few pounds which I am now trying to shed because it is higher than I would like to be. Either way I hope that it levels out for your daughter and that whatever the cause she is able to come to a happy and healthy point in regards to her body image.
Lindsey