According to a body mass index chart I consulted, the girl 5'7" @160 lbs. Is average--her bmi is 26. That's what most people in this country have.
The ideal bmi is 23 or so, wjich would be about 145 lbs. In her case. But everyone's different and she's better off staying at that weight than going on some crazy diet and maybe screwing up her metabolism & eating behaviors.
If she exercises a little, walking 30 minutes daily, and watches the food (sweets, fried foods, junk food snacks) then she will probably naturally lose a little. But I caution against any drastic measures, as long as she is healthy & happy!