Can't Put My Finger On It

I've been having trouble with a friend of mine, so I talked it over with someone who knows about these things. It's not the usual kind of trouble that one normally deals with in relationship. This person said something that surprised me -- he said he thought there was a personality disorder going on here. I kinda knew what that was but not really, so I looked it up. It's probably not a good idea to be diagnosing people around us, but this description fit the situation pretty well.

A person with a borderline personality disorder often experiences a repetitive pattern of instability in self-image, mood, behavior and close personal relationships. This can cause significant distress or impairment in friendships and work. A person with this disorder can often be bright and intelligent, and appear warm, friendly and competent. They sometimes can maintain this appearance for a number of years until their defense structure crumbles, usually around a stressful situation like the breakup of a romantic relationship or the death of a parent.