You should be concerned.
Having sexy conversations with a co-worker is the gateway to having sex. Whether or not he does have sex with her, it’s not what a committed husband does. If he’s going out for drinks with his buddies or even for work, that’s pretty normal, but going out for drinks and staying out overnight is a flashing yellow light — especially coupled with the social media convos.
Chances are this behavior of his didn’t happen in a vacuum. If you want the marriage to work, figure out what he’s getting from this woman that he’s not getting from you. Men don’t cheat because they’re attracted to someone. They cheat because they like the way they feel with someone else because it’s a feeling they don’t get in the marriage — whether it’s respect, fun, sex — whatever it is, figure out where the crack in the marriage is and work to heal it. You don’t want to end up playing “whack-a-mole” with women he’s flirting or cheating with. When you figure out how to deal with one, another one will just pop up. Instead, address the problem — not the symptoms.
Let me know if you have any other questions.