I'm honestly not sure what to say about how to deal with the calling and texting of the EA. He's seen phone logs and spied on texts, and knows it's more than friends. It reminds me so much of where I was went I took my son away with them during the period right after my BD where I knew my then-W was texting her coworker inappropriately. But...does he tell her to stop? He can't control her. But if he doesn't does he then just go along and accept it? That's weak. She knows he knows she's talking to a "friend" and they both now at a minimum it's crossing a line. They've talked about it. And he's had check ins with her about it, which I told him to stop.
I'd tell him to stop pretending everything's okay. She's cake-eating, enjoying him while trying OM on for size. She's a human being, and his co-parent, but not currently his partner. Avoid venting, proclamations, and attempts to control her. Do set boundaries around his own behaviors. E.g., does he want to sleep with someone cheating on him? Does he want to sleep next to her? Begin thinking about what's best for him and the kids instead of what's best for their former family unit. Talk to an attorney to protect his finances and custody. He sounds like someone who needs to find where his sack is.
Take the energy he was putting into her and put it into his kids and GAL and 180s. If any of this triggers her to talk about the issues that led her to want to leave the relationship, which likely predated the affair, LISTEN AND VALIDATE. If he agrees, consider adding those as 180s, maybe to save the marriage or maybe for the next relationship he has.
Get into IC (or chat with you more) to work through his feelings. So many people make rash choices in these situations fueled by either anger or fear. It's incredibly challenging when you don't fully control the outcome.
