My best friend doesn't believe my bf and said a real best friend would still be her maid of honor Update

Update from my post yesterday but the gist of my last post was my boyfriend told me my best friends fiance sexually assaulted him. When me and my best friend were looking at her dress downstairs the boys were upstairs and my boyfriend said her fiance kept getting behind him to touch his pool stick and her fiancè stuck his hands down his sweatpants and boxers and rubbed his butthole and taint. My boyfriend was gone when we came back upstairs. My boyfriend was just discharged from the military for medical and mental health reason and already has PTSD. I told my best friend today and she was quiet and then said my boyfriend had to be lying and her bf is straight. She wouldn't let me get a word in and said he's a military vet and vets are crazy and see things. She told me she never wants to hear this again and to shut up and if I'm a true friend I'll have her back rigjt beside her at her wedding. She refused to let me talk. I've been trying to give her time to cool down. I'm hoping she will be open to listen once she's calmed down.

Update: My best friend called me right before work and I answered and she said "Everything's fine now! I talked to him and he said he meant that as a joke". I meant to say this in my head by I said it out loud "Joke? Can someone explain the punch line because nobodies laughing". She's saying her fiance was joking around with him and didn't know it would freak him out so much. He doesn't even know my boyfriend. That would freak most people out! She is saying since he said he meant it as a joke it should be let go and she doesn't seem to understand why that's not a joke