I endorse Bears comments, but will add its very much down to your daughter and how she is, manages thinks and copes with her own body image. ALL younger girls appear to moan about being fat, its a peer thing and yes its from the media, pop culture and even vile rap video with virtually naked women.
My daughter is 21, she seems to be suffering a bit more than me shape wise, she is pear shaped but with few pounds on top too.
I told her before the surgery but only a week or so before so she didn't make a fuss or worry, i know its the best way to handle MY daughter.
She was worried when i was out in Prague but i kept in contact with her, she is happy i am happy and i think one day she will end up doing the same. We don't seem to have very good metabolisms and genes! I would support her if she did but only if she tried non-evasive avenues first and she changed some of her own habits and lifestyle a bit.
She is a bit of a lazy bum and i know she would be thinking its a problem solver, we all know its not....... its a tool to assist our efforts WE have to make
your call based on your daughter, if you tell then, soften it as suits her, timing etc well again before or after its how you know she will react
good luck xxx