The important thing here is that a man frightened a vulnerable girl ~ forcing himself upon her, even when she was saying 'no'. This can never be acceptable.
As her friend (the fact that you want to be her boyfriend is secondary here) you need to talk to her and warn her that this kind of thing may be repeated. If he will do it once, then it is likely that he will do it again. This is a common pattern, as far as I am aware from the testimonies of other women.
Women often do love their abusers, which is why they forgive them, but she is in the early stages of the relationship. She can walk away before anything more serious happens.
Talk to her about this. Tell her that it is not jealousy on your part, it is concern.
Good luck!
