9 yr old girl Masturbating

Abeni is 9 years old, and was sexually abused at age 5. Then she taught sexual behavious to other children in the home before she was removed from the home. She does not like to talk about the incident. Now, whenever she moves to a new home, she'll masturbate...most likely to comfort herself. If she's esspecially uneasy, she'll masturbate to the point that her vagina will be sore. Her current foster mom tells her that its bad to do that, but they'll work on it together. Shortly after being told that she started hiding her panties (they had a discharge on it). Now, i don't agree that masturbation is bad. However, it is unusual in a 9 year old. BUt its not unusual in a 9 year old who has been sexually abused in foster care. She doesn't exactly know what she is doing. She barely knows the "Birds and the Bees", and still thinks kissing is nasty. I don't think that I, as the adoptive mom will try to discourage it. She doesn't do it in public. I'd like to encourage her to talk about it, but given that she gets proper medical care...is there any reason that i should try and stop her? considering that, in her teens, i'd turn around and tell her that masturbation is okay?