When do angry words become violent? Is there a correlation?

Hi
I'm writing this on behalf of a close friend of mine who is terrified of her boyfriend. They have been together for about half a year, and in public he is extremely dedicated and sweet (although my cynical side always told me.. and I don't mean this with even a slightest ounce of bitterness of jealousy... that he was trying a little too hard, as if he was putting on a show). Obviously, she fell in love with him very very quickly (soon after they met).

I'm one of her closest friends so she confides in me on things that she would normally not tell anyone else. Lately, she has become terrified of him and she's unsure what to do next. I think breaking up is easier said than done, because even when she is incredibly upset with him, she gets reminded of some of the sweetest things he does and then she gets confused.

The problem that she's dealing with is that he could be perfectly content and happy and charming and caring, and then something will snap and he will get extremely angry. And when he gets angry, he tells her things and then supposedly forgets that those words were even said. I am not sure if he actually does forget, or if he's pretending to forget, but the things he says are extremely disturbing.... and getting worse with each subsequent episode of anger outburst. It started with him saying extremely harsh things that would really twist the knife on some of the most insecure things she had told him about her. Then it grew into something violent - he would say things like "If I did that, I will grab that knife and chop off all my fingers one by one!" And then lately, he started threatening her. He told her he would stab her. He told her that he regretted not pushing her off that cliff when he had a chance. Recently, he told her that he wants to run her over with his car and then run over her body over and over again... and then not feel a thing.

Obviously, these are disturbing things to be said, and something that should never be said to anyone even if you are extremely agitated. My friend - I'm not sure if it's because she is extremely kind or extremely insecure - while she acknowledges that this guy has a problem with anger, she is willing to believe him when he says that he has no recollection of things he says while agitated, and wants to work with him. I do not know what to do here - I know it's not my business as it is her life, but should I be concerned that any of his words might one day be executed? Does verbal abuse ever become physical, or are these empty threats?

To be honest, I do not trust this guy completely. My gut feeling tells me that this guy is always putting on a show when he's playing the "wonderful" boyfriend. For instance, there was a time when he picked up a bunch of flowers from a grocery store for her birthday... he milked the story so much that you would've thought he went into the amazon jungle and fought off the wild beast to get those bouquet of flowers! My friend is fairly insecure and he's very manipulative and charming, so I think he knows exactly what he is doing.