Do i have the right to be angry?

Okay so there is this psycho that is chasing after my girlfriend, he has been at it for a very long time give it, say 3 or 4 years although her and I have not been dating that long, but he sends her a text every night saying how much he loves her. She blows it off, tells him that it is never going to happen he replies by saying, "oh it is okay love you will come around and realize I am the only guy for you" my girlfriend says she is used to putting up with it and doesn't even care about it anymore, but it is starting to get under my skin... he used to be a decent friend of mine. I wouldn't say a best friend or anything, far from, but someone to hang out with. But my girlfriend means so much more to me than this guys feelings, I would hate to see him come unhinged and do something aweful to my girlfriend, she really does mean a lot to me. has anyone else dealt with psycho stalkers? any advice? I would hate for things to get violent, heck I don't even know if I should intervene! help please :(