OP, some people spend £25k on weddings - well, your DD thinks there won't be one with the OP
MN are obsessed with home ownership; sometimes it's not the top priority, or even any priority
Finding a partner in plenty of time to start a family, if that's what she wants, may be more important to her happiness.
Her body, her choice - if it's her money
imo since you don't have the money, you shouldn't take out the loan yourself
However, do support her in other ways,
e.g. emotionally - no criticism,
also consultations before surgery, journeys to / from hospital, aftercare, maybe let her move in with you for a couple of years to help save up.
Is her weight stable after 2 years, or is some gradually coming back ?
If she is stable for say another year, then it's a reasonable gamble that she has learned the new habits she needs to maintain.
A reputable surgeon should explain what scars she would have, where and how vivid they are likely to be
and give her time to decide if she will accept that.