my friends mad at me and i dont know what to do?

ive known her since 4th grade and shes the closest of my friends (ironically im fighting with all 3 of them at the moment!) she used to get mad at me for talking about my boyfriend cause ive been with him for almost 9 months and she wasnt used to any of us having long term relationships, she got over that eventually because now me and the other two do, but she doesnt.she recently started dating, but i know for sure that the guy shes dating is bad news. everygirl he dated he used for sex and cheated on, i know because my cousin dated him.he knows i know to, and he told ,me that if i say anything to her about it he would make sure my life is a living hell, idk how he would do that but i dont wanna underestimate him.should i tell her about these threats, my boyfriend thinks i should, but this is the first time shes been happy in a while, is there a high chance he changed or is it once a cheater and player always a cheater and player? shes been mad at me for not being happy for her ( i havnt said much about them dating and try to change the subject when she brings him up) should i warn her of what i know and tell her what he said to me