How much weight does she need to lose?

Her problem isn't her weight, it's her body type. See the girls on the left? They have no hips. It looks like they haven't even hit puberty yet. The girl on the right, though, has hips, which is why she looks bigger. I don't see how losing or gaining weight will make any kind of difference in this scenario. Just wait a year for the girls on the left to grow up.

No weight because she is beautiful just how she is.

She doesn't need to lose any weight per say but if she wants a stronger definition she's going to need to do ab workouts. Not much but maybe 30min every day. There are a lot of east workouts. However these pics r a little skewed as @BenderIsGreat mentioned their ages and body types are quite different. Plus the left girls are tensing and standing up straight, so posture is just as important.

(agreed with bender) but if the aim is a flatter stomach, then probably maybe 2-3 pounds. Honestly, i think the body is perfectly healthy already :/