She is playing him like a fiddle, I have instigated EA with married men before in my younger days, when I was unhappy and looking for some attention.
Men are so easy to play, you fluff their ego, you tell them they are amazing listening to you "go on". They are a great listener, only they seem to understand. You flirt and flutter your eyelashes and they open up to you about little things at home.
Before you know it your "helping" each other out with home life issues, you become their focus, they are oblivious to those around them noticing the two of you, because they believe they are being discreet. They aren't!!!
It impacts at home, because the focus is on you and not their wife/partner. They message you, check out your social media, take annual leave to meet up or have "meetings away" as cover.
It becomes a full blown affair before you know it.
He likes her a lot all the signs are there, they are having an EA, and would move on to a full blown affair given half a chance.
Work has noticed, he is minimising and using damage control.
He doesn't need to follow her on SM or contact her outside work. Tell him no and mean it