This ticks all the boxes for infidelity.
It's secret, they fancy one another (or rather they fancied eachother when they were young and despite what is probably contradictory photographic evidence, they still 'see' eachother as then) and they are sharing intimacies.
This is such a depressingly common situation. She's going through a marriage break-up and decides she needs her ego boosting, so she chooses the safe bet of someone who once liked her. She 'friends' him and they start talking.
He tells himself that despite you having absolutely no form for jealousy, you will react badly to him being in contact with Ms. Saddo, so he keeps schtum. The truth is he wants to see how far this will lead and is enjoying himself.
He pours the sympathy on with a trowel and simultaneously stops mentioning you and how happy he is in his status updates. If she asks about you and your relationship, he underplays it.
She thinks oh what a lovely man, so unlike my ex husband. I bet his wife doesn't realise how lucky she is! Grrrr...and he doesn't sound too happy either. I bet it's a loveless marriage and he's only there for the kids.....so I'll start complimenting him.
He thinks....oh poor Ms. Saddo. Her ex sounds like a bastard. It's important I show her that not all men are like this. I'll build her up a bit and tell her she's lovely and any man would be proud to be with her. I must admit I feel ten feet tall when she says all these nice things. All Clolan does is bark orders and get pissed off when I'm on the internet all the time......
That my love is the script. He can tell you and himself that he 'doesn't know' why he kept this a secret, but he's lying.