I don't know whether or not you did the right thing. It depends on who you ask.
I personally question your motives for telling the boyfriend. It sounds more like you were the jealous ex-lover trying to break them up than the guy who was trying to do the "right" thing. As you can see, your interference wasn't welcome.
I wouldn't have advised you to tell the boyfriend at all had you come and asked beforehand. I would have just suggested you stay away from the girl. Their relationship is none of your business. Other people will certainly disagree with that.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?