I think I’m seeing a pattern ~ you have a lover, and even though he doesn’t know about it, he was picking up on some energy i.e. “What is making her so happy??” and then he became interested.

He needed “Nice and Easy”/some new energy, and he is all creative/expressive/emotional with the new girl.

And he was more doting to you at one point ~ maybe he met her around then, and he was more loving because he needed an outlet or because of guilt.

And I do agree that him taking her into YOUR home was a violation!! Wow! That was aggressive. She must have picked up the Clue Phone that this was wrong and wisely LEFT!

I’m not big on ultimatums, but I would say to him, “We are obviously incapable of handling this on our own. See a counselor with me. If not, I’m calling my lawyer.” THAT will scare him! The opening up in front of strangers is a cop out. His colleague was once a stranger, after all.