It would help to give some more detail about where she is now. But on the other hand, bf is just a number. 20% on one person can look great but on another person can look like flab. Also don't get caught up in comparing women's bf measures to men's. I dieted down to 8% for my first natural comp but still didn't get much cut in my legs and my ass wasn't tight. But I had a nice xmas tree on my back.
If she tends to carry her weight in the caboose, no matter how much bf she loses (short of lipo), she will prob always be fighting the caboose. I have friend who is an inch taller than me, but 10 lbs lighter, who is 23% bf. I'm shorter, weigh more and have maybe 18% bf. She looks leaner than me - but then I have more muscle. So its all relative. The average female bf is around 25% I think.
I think the bigger thing women worry about when losing bodyfat is losing their breasts, not their butts because they will most likely generally carry less fat on their upper bodies, and since you lose bodyfat proportionally all around your body, end up appearing to lose more on the upper body than lower.
If she really wants to get lean (e.g. off season bb), a reasonable maintenance bf is prob 16-18% bf. Below 13-14% is hard to maintain year round and below 12% she'll probably stop getting her period - NOT a good thing for extended periods of time.
If she's just starting to train, she should just shoot for the point where she is happy with her results - there is no number where she "should" be.