No no no! Not only is she not done growing but she's a child, that's awful behaviour to go behind your back.
I've always said I want a breast enlargement from early teens, I'm now 24 and although I still like the idea, I'm very much on the fence and really quite happy with my not so impressive breasts.
Unfortunately we have body trends, and trends change. Small boobs are 'in' now, she may regret it.
Look into breast implant sickness too, it's emerging that tonnes of women are suffering with it. Get her to look on YouTube there are alot of women taking about it.
Complications with breastfreeding, capsular contraction, getting them redone, having complications and having to have them removed likely needing a lift and the scars that come with it.
I still want bigger breasts and to fill out tops, but is it worth potentially suffering with the above, I'm not so sure...! I'm going to wait until I'm finished with dcs and bf then see how I feel.
Also worth mentioning, I've had some hormonal issues resulting in very engorged sore breasts and changes to my nipple, I couldn't imagine the extra discomfort implants would have!