Most Helpful Guy
In order for a relationship to be healthy, there needs to be TRUST. He just got out of a relationship, so it could be possible that he didn't do anything. It is possible that he lied because he knew you wouldn't understand. It is also possible that he accidentally did something with her. But I doubt it is that in this case.
It does not matter that he is 21 years old and already out of marriage. For all you know, he could have learned from his mistakes. But it is hard for us answerers to know more details without actually being observers.