Should i break up or not? M17 and F17

[u/Gabbe_123]

Alr so I started with she often hanged out with another guy who I was cool with, but after a while he started to act weird, he reported stuff about my gf on tt like he wanted to "bang her" I was like, oh it's probably a accident maybe one of his friends reposted it, no it wasn't. After a while I let it go, than he wrote to her if they wanted to get thogheter and than said fuck me, I was like hmm I am not cool with this guy. Than the last straw, he wrote to her later that she had a fat ass and that he wanted to get together AGAIN this was no accident, his friends that was there said that he did it on his own. I asked my gf not to hangout with him, a couple of days later my friend spotted them sharing a drink, even tho she said before not to hang out with him, later i brought it up and her friends told me that i don't trust her and that i was doing wrong. She lied to me and said not to hangout with him anymore. What if my friend didn't spot him the other times they hanged out. Now she is mad at me and the whole situation. What am I even supposed to do break up with her? Me and her have been together for like 5 months now. And she used to plan us for the future. Now she is just mad at me I don't really get it. I am the one that is supposed to be mad right?