Technically it's "child porn," and I would warn her of that.
Teenagers have been prosecuted for possession of child porn for sending images of themselves flashing their chests to their boyfriends, or for receiving those photos.
The way I look at it is, if I had a friend from out of town and we were driving down Lake Shore Drive, and I noticed that we were going 55, I would inform them that while this looks like an expressway, it's not, and the speed limit is 45. If I had a friend from another country where seatbelts weren't mandatory and we got into a car here, I would inform them that buckling up is the law. After being informed, they can do as they wish, but I would prefer to assume the Facebook friend is ignorant of the law rather than that she's distributing illicit photos.