Co-parenting with a narcissist

I'm not sure if I'll get any sort of help but it's worth a shot...

My significant other and I have been together for a few years now. He has a 7 year old from a previous very short and what he describes as toxic relationship. And he's having trouble co-parenting with the mother of his child.

She's pretty much what I'd describe as a narcissist. Nothing he's ever done has been "enough", she's constantly yelling regardless of where they might be or who might be around (their child included), she is constantly threatening to take him to court for full custody. She has been so verbally, emotionally and even psychically abusive towards him. She's super manipulative and will pretend to be nice one day and then throw her niceness in your face the next. He's been so worn down, he's tried everything, begged her to go to counseling with him, tried to do everything she asked and wanted to a T. She even wore him down to the point he dumped his whole life and tried to make things work in a relationship with her as a desperate effort to have some sort of peace in his life and be able to be with his son. That lasted like three days...

He gets so torn down by her. He's been struggling with depression and anxiety to an extreme the last few years and he's made some great progress but he has days where he's so defeated, he just looks at me hopeless and teary eyed and says "I hate that this is my life..."

Court won't do anything because she's the mother and legally has a clean record. She refuses therapy or anything of the sorts with him or alone. I've tried to be the buffer which only lead to her abusing me and him. Neither of us know how to navigate this anymore... Even if it's just meeting someone to drop off their child it can blow up into this huge thing where cops are called because some woman's screaming at a guy in a parking lot... It's starting to seem hopeless and I know it's not necessarily "my place" and I don't want to step on anyone's toes but maybe someone has a suggestion on how to handle this. :/