stupid mistake

Ok, I don't know if this has been posted on here before but anyway.There's a part in this movie that is really stupid.The girl with the red hair( can't remember her name) makes up with her really bitchy friend at work.The same day the psycho roomate calls her and invites her to dinner.She does this before the red headed girl got home and had a chance to tell her that they had made up.Even if she had been home, they were hardly even speaking to each other by then so I doubt she would have mentioned it anyway.So there's no way she would have called her from work to tell her either.Especially since she knew she was really weird and gets really jelous and doesn't like the bitchy friend.Yet the phsyco calls the bitchy friend up to invite her over to dinner pretending to be the red head.That's totally stupid because as far as the roomate knew they hated each other.Why did she act like she knew they had made up.The phsycho was totally friendly on the phone and didn't even try to makeup with the bitchy one to try and lure her over for dinner even though as far as she knew the person she was impersonating hated her.