She can come into heat anytime from 4 months on, depends on her. It is usually around 6 months. If she is not in heat, unless its a silent heat, then he can't breed her although he can mount her. He obviously did not penetrate her or tie, he would not have been able to get off her that quickly.
I suspect it was a typical young pup going through the motions and your girl, because she is youngster just accepted him doing his tryouts on her. If your girl had been a mature female, whether spayed or not, she would have taken him to task and taught him NOT to mount females unless they were ready to breed.