I would tell your daughter that you aren't taking this away from her if it is really important to her. Tell her you guys are "postponing" it. I don't think that it would be medically wise for her to do it until she is in remission. Once she gets into remission and with the doctors OK. Go for it! =0)
I can feel her pain. It's not fair and it's the one thing that you are hoping for that will make things seem like they're livable cause everything else is so crappy. But if she's not in remission and she gets complications, not only will her bottom hurt but so will her boobs and armpits. I got a breast reduction when I was 25 and was in perfect health and it still took a little for me to get back to normal. Tell her that she's not giving it up, she's just postponing.....
p.s. II your boobs do get bigger as you get older. They definately got bigger between 19 and 21 and even bigger when I was 25 and I was a size 7. I realize not all bodies are the same but there you go.