Re: This feels like a soap opera
Whether or not the school can be sued, this is the kind of epic PR disaster the District Superintendent is going to pay complete attention to. You have two married teachers carrying on a sexual affair at school, one of whom was even pregnant at the time! The assistant principal gave one of them advise on how to continue the affair. I didn't catch whether her "friend" who is also having an affair works at the school too, but if she does it is just more ammunition for the firing squad.
If you explain what is going on to the principal and they do not take action they will lose their job when this becomes public. It is that simple and if they don't see that they deserve to lose their job.
Really important: Do not let on that you are accessing your wife's school email account. That is potentially not legal and could be used against you.
Whether you inform their employer, I think informing the OM's wife and the husband of the monstrous "friend" is a moral duty you need to fulfill. They need to know what their spouse is doing, it literally is putting lives at risk and that obligates you to act.
I do think it could be helpful to go talk with the principal, they have the ability to keep your wife apart from the OM at school and you don't need to go into details about things to demand they take action. That may mean firing one or both of them, it may mean threatening them with firing if there is any contact between them during his remaining time working there. If nothing else, the principal should be aware of the coming disaster your wife, the OM, and the assistant principal have created for the school when word gets out.