Was it wrong of my boyfriend to tell my "ex-friend" that if she hit me again, he would "strangle her?"?

Well, yes and no. In the heat of the moment it is understandable that he would want to defend you and communicate that in a way that expressed himself poorly. Such as the threat to "strangle" her. Strangling is a means of literally killing someone, which would be a bit of an overreaction unless the girl was intending to kill you, which she probably wasn't. It would have been better if he would have said something like, "you have no right to assault my girlfriend, if you persist in this effort I will use the minimal level of physical force necessary to ensure her safety first, and your safety secondly and after having done so will call security or police to handle this situation appropriately". With her barging into your remain it is understandable that he was not able to put his intentions into those or similar words, given the suddenness of the situation. Probably, though, that is what he meant and it just came out as a bit more violent than he really intended. When people ask about it you could explain the situation by telling them the circumstances and what he really meant to say and saying something like "My boyfriend and I regret the choice of words he used to express his intentions of not letting whats her name assault me. He only intended to keep her from hurting me and had no desire of hurting her. Given the suddenness and surprise of the situation he was didn't state things exactly as he meant them, which is quite understandable."