Please every other reddit cheating story is a man in his mid/late thirties snd a 20 something girl. You should be concerned. I would stress he needs to stop it now - it has crossed lines - He needs to be blunt and say - I am happily married and love my wife . We need to only message about work. They don’t need to be buddies at work - Work is for work and polite conversation - not hanging out snd messaging about other things dll day. He needs to go back to being strictly professional and she can have friendship with people her own age outside of work who are not your husband.