Why do guys cause so much trouble?! helppp?

so theres this guy that kept telling his friends that he thought i was "the most gorgeous girl he's ever seen" and that he really liked me and all this stuff. last night, we were supposed to hang out, me and my friends and him and his friends, but i couldnt go. so, him and his friend just hung out with my friends and one of my friends brought this other girl that is probably the biggest loser in the entire school (i dont know why she brought her). apparantly, he was talking to this girl and decided that he liked her and he got her number and all this stuff. but the thing is, shes REAALLY ugly. not to be mean. but shes just not pretty at all in any way. and it cant be her personality because shes just really loud and obnoxious and people never seem to like her. she has very few friends. and she knew that me and him had a thing and she was all over him anyways. ewww i hate her. do you think that he really likes her or just wants to get some? bc thats what he was trying to do i guess. should i be more mad at him or yell at her. i kinda wanna go and talk to him (because if i actually talked to him and acted interested, he would drop her so quickly) just to rub it in her face and then dump him and never talk to him again :) what should i do?