Agree coworker is definitely interested but.... they are coworkers and I'm going to assume she knows he's married. Maybe I'm naive but I would not be surprised if there was no move past flirting. Everything OP describes sounds like husband and coworker walked the line of "plausible deniability". Definitely not ok. Definitely heading down a bad path, probably already over an emotional line.

Honestly, If I was in OP's shoes and this was the first time something like this came out I would give him the benefit of the doubt, so long as he makes the effort to fix things in the marriage going forward. That changes if he has a history of looking outside the relationship for emotional validation or if there is a history of cheating. And if something like this comes up again he doesn't get any more benefit fo the doubt. But I don't think anything good comes of reading what will most likely be vaguely flirty, but not really definitively anything conversations or seeing that he called her occasionally.