If your H is a supervisor, and having an A at work, they can both be fired. If it turns out that they used company time to have a relationship, company phones to talk, company computers to email - they can both be fired. It doesn't matter if he is at a sister company, another state, or not having a relationship now. All she has to do is complain or tell others what happened in the past, who then complain. They can lie, too. They can say he was harrassing, it was unwelcome attention, or it made other people uncomfortable.
Company emails and phone records belong to the company. They can investigate at any time, and go back through their records. They can read employee emails. As long as he works there, he can lose his job and his chance at having a referrence from this job.
He is much better off leaving and finding another job where they do not work together. He should do that NOW, immediately, while he can salvage his reputation. Is he doing this?
Is he being open and honest with you? Do you have access to his email, phone, bank accounts and whereabouts?
Having a secret relationship for two years while you are marriage is an A - EA or PA. I am skeptical that a two year relationship wasn't a PA, though. I think you are better off preparing to deal with the fact that it was an EA and a PA.
As for protecting the OW, please don't. She is an adult. She chose to have a relationship with a co-worker (never a good idea), who is married (always a terrible choice). If I were her employer, and found out she was having an A on company time, I would let her go immediately. People need to experience the consequences of their decisions. Getting away with wrong-doing leads to justification of it!