Is it normal that a girl wants to fight my sister for not liking her bf?

My sister and I used to have this guy friend who we now don't talk to because he was too annoying, a know it all, sensitive, and clingy. He used to have a crush on both of us but we never liked him back that way. He then got a girlfriend who is friends with another friend of mine. That friend of mine told me that his now girlfriend said she wants to beat my sister up for back then, not having feelings for him or liking him back when he liked her. So apparently he told his girlfriend about how he liked my sister and her not liking him back. What the heck? Crazy psycho bitch right? We don't even know her or go around her and their little crazy group so I'm not sure if we should just ignore this, or just what to do. This pissed me off when I found out. Please help!