Just confused.

My guy best friend stopped talking to me because I said that his new girlfriend is a bad person, which she is. She keeps passing me notes in my 5th period threatening me and I told him he won't believe me and says I'm lying. He wants to believe her but she cheats on everyone and I just wanna help my friend. I usually sit next to him in 2nd period but he asked the teacher to change his assigned seat and now I'm just alone. He was one of my only friends and he was a great one. I don't know how to tell him that it's a toxic relationship without him hating me any more then he already does.