Gaining

You can't control where fat goes, but she could try to build up her glutes and thighs with elliptical or stairmaster machine at the gym, to get bigger in those specific areas.

Wearing white/tight clothes on bottom and loose/dark on top would also help make her look more bottom-heavy.

If there was a way to get a bigger butt, most likely all large people would do it, since belly fat is considered a health risk, and butt fat is considered a cosmetic problem.