If they are casual acquaintances, which is the case 98% of the time you treat them the way you would if you move to a new job. Promise to stay in touch and send them a Christmas card for two years and then stop.
In this case they are on shaky ground morally because he is married and they are having a romance even if they have not yet had sex.
Once their relationship moves out of being in public all the time they will tumble into bed.
There is a lady I worked with that my wife refers to as a work wife. She had four brothers and was the only girl and in my family there was four brothers and an only sister so we hit it off that way. We treated each other as siblings, picking at each other and playing jokes on one another. She’s married and other than at work we never see or talk to one another except for 5 minutes to buy chicken eggs from her. I’ve never looked at her as a love interest, just another sister and treated her that way and she me as another brother. She loves animals and has chickens, goats, dogs and cats and I have nicknamed her Ellie Mae.
One of the best jokes I ever got on her was at a Mexican restaurant. Her and another lady I work with share a birthday and we went out for lunch for their birthdays and at this Mexican restaurant there is one particular Mexican manager that has the hots for Ellie Mae and as the lunch proceeded I watched him just sit and observer her and I commented to both of them, Ellie your admirer is admiring you sit up straight and be lady like and she proceeded to kick me in the shines under the table and say oh shut up! On the way out as I was paying he tried to engage her in conversation and feeling the need to get some payback for the shines I asked her if she needed some matches, they had books of matches and she said no and I said I know you do because you are always looking for a light in the evenings when you sit around and watch tv in your underwear and smoke cigars! The Mexican managers eyes about popped out of his head and she said I’m gonna kill you and as we walked out did the obligatory punch to my upper arm and you could hear him saying bye my mamacita! The other lady just giggled when he said that and Elie Mae didn’t think anything of it.
When we got back to work in about an hour I had to go take something to another department and I walked by her office she shared with another employee and stuck my head in the door and she was red faced and I said hey you still mad? Not at you, your a jerk still but I would have slapped that SOB if I had known what my mamacita meant! She had googled it. I just laughed and said see ya momma and walked off.
She got transferred to another campus in our system 15 minutes away so I hardly see her anymore. I buy eggs from her now every other week and see her for a whopping 5 minutes.
You can be friends with the opposite sex at work but you must be cautious. With Ellie Mae sexual attraction just never entered the picture even though she is a pretty woman, we had fun cutting up like brothers and sisters but I could see how it might with others.