Long story short, I (21F) became best friends with a senior girl when I was a freshman, let’s call her Lisa. I was in this friend group from my dorm, including my freshman guy friend, let’s call him Kyle, and his best female friend from their hometown, let’s call her Kelly. Kyle thought Lisa was very attractive but he was too young for her as she was a senior and he was a freshman. But our sophomore year (she did a fifth year) she agreed to date him after some bad dating experiences with guys her age. He started cheating on her pretty soon, nothing crazy but flirting and dancing with girls, getting their snapchats, talking to his ex etc. I told her and he denied it, and his close friends denied it too because they always cover for him, so I looked crazy. This situation happened quite a few times, where he would do something and i’d hear about it or she would even see it, but he would manipulate her and I would look bad for snitching. Kelly (Kyle’s hometown friend) and I got very close but I ended up kind of leaving the group because of how unethical they all were. We hung out again later when we were juniors for Lisa’s birthday and Kelly told me Kyle had sex with another girl. I obviously had to tell Lisa, but Kelly was angry I threw her under the bus since she was my source. Again, Kyle and everyone denied it and I looked bad. Since then I've left it alone and Lisa and I stayed close, but it's hard to see her become so committed to this cheating frat boy. Kelly and I stopped talking and I tried to make amends but she ignored me for 5 months…until last week. We ran into each other at a bar and turns out she had no idea Kyle and Lisa were still dating. He has a new girl, has been telling everyone he dumped Lisa, and has been talking bad about me to keep everyone away from me so they don't find out and put everything together. Kelly was enraged when we figured out what he's been doing, the lengths he's gone to and the lies he's told to his supposed best friends. He is a sociopath. Kelly is too loyal to him and too non confrontational to do anything about it, but I have to. We just graduated and Kyle and Lisa are planning to move in together. I've told her so many times before that he's cheating and she never believes me, so how can I get her to believe this? I have no actual evidence. Plus i’m scared of Kyle, who knows what he’ll do if his web of lies falls apart.