Husband doesn't want me to argue with his ex

My husband's ex-girlfriend has been talking about him online. She did horrible things to my husband, one of the things she did ended up in court. She confessed. Signed an affidavit her statement and confessed in court what she did which kept it from going to trial. This isn't me just hearing his side. Everything is legally documented. They've been split up 9 years and she randomly has decided to talk about things and how my husband ruined her life and is manipulative, narcissistic liar. Coming from the manipulative narcissistic liar. I have wanted to say something. My husband's own friends have said something. My MIL has wanted to say something but my husband doesn't want me or his mom getting involved. He didn't want his best friend getting involved either but said his best friend was going to do what his he wishes if he thought it was helping but he wants me and his mom to just be there with him and stay out of it because it causes him more stress. Would you be able to keep your mouth shut when someone who caused your husband so much pain and out running their mouth.

Edit: I mean I'm trying to respect his wishes. But he isn't telling his friend to do the same.