I have to agree with what rob and audury(i'm pretty sure it was you two...lol) said.
However, I know I'm only 2/3 years older than your daughter, but I really believe that under 18 is a little too young.
I mean, there's no way I would have been ready for this at 16.
Heck, I never even thought about it until a few months ago when my father mentioned it to me(he had a gastric bypass at 30)
I mean, I think most kids are looking at this as an easy way out
Like, get the surgery, loose a bunch of weight over night(which is not the case) and magically it will completly change their life.
I'm not saying that it isen't something that should be looked into though.
I would really have to suggest finding someone that your daughter can talk to, because it helps alot.
And, I'm not sure how much she's tried dieting or whatnot(i've been on too many too count)
I know something my mom did to try to encourage my weightloss was to reward me with things I wanted. So, I mean, say if she looses 20lbs she can get something she's wanted. I know for me it was piercings(which my mother hates) but if I worked towards it, she let me get it, even though she didn't like it.
I'm just saying this because, I really think that surgery should be the last string. I know for me though, that this is what's it's going to have to be.
However, all that being said. If in a year or so she still really wants it, and hasen't been able to loose weight, I say go for it. Why wait till long term medical effects take thier tole.