There have been some great resources posted already but I just wanted to say that helping him connect with these could be very powerful in shifting how he’s thinking. And his mindset is the key to ending this.
I disagree strongly with the posters urging you to act, because there is a strong likelihood that he will crumble at this point, that she will win the emotional battle and he will end up even more isolated if he goes with her. I’d keep your powder dry a little longer.
Speaking to a men’s support group, he will have to tell his story, and will be validated in doing so. You can assure him that he’s not calling in the cavalry - he’s still dealing with it by himself - this is just about exploring options and listening to advice. You can reassure him that he still can decide what to do.
Take him somewhere safe to call. I’d suggest leaving his phone at home (he can call from yours) and sit with him while he tries to get through.
I’ve seen it over and over on the relationships board here where posters come on to talk about an incident that they’re struggling with and it’s in the validation they receive here that they start to understand the full extent of their situation. Our brains react to danger by protecting us - sometimes we can’t see the bigger picture. Telling the story to people who understand, changes our understanding.
But one thing he needs to know is that ending a relationship is the most dangerous time. For him and for you. If and when he’s ready to end it, there’s no preamble, no warnings, no discussions. It has to be fast and decisive.
He needs, to borrow a MN-ism, find his anger. Get him talking to one or some of the resources posted above and go from there. Good luck op