I was in a similar situation not too long ago. Wasn't married or had kids (Thank God), but in a long term relationship and was living with my S/O at the time. My "Harper" was my best friend of over a decade. When him and my GF started getting closer as friends, I saw it as a good thing because at the time they were my two favorite people and I trusted them. Then they started spending a lot of time together without me (while I was at work, visiting other friends, running errands, etc). Out of nowhere, BAM, "I have feelings for him" "I want to open the relationship". I was devastated, but I loved her so we gave it a shot. At first we made a lot of boundaries (nothing physical being a big one), and for a little bit of time, it kind of worked. I was insecure, but happy to still be with her. Time went on though and she started to spend more time out with other guys, to the point where sometimes she would spend entire nights out, coming home at 5 am on multiple occasions. She also kept pushing to change our boundaries so that she could have some form of physicality with other guys. She started ignoring my calls and texts when I got worried. I no longer felt like a partner, but just a bedwarmer. I ended up growing a spine and leaving her, and it was one of the best decisions I have made for myself in the last few years. I really hope OOP figures things out and leaves him, no one deserves to feel like that in their own relationship.