She said "why did you tell her about me"

My bf was talking to a coworker and was on some cheating shit. He says nothing ever happened in person but he was texting her things like i wish you were here and basically telling her how bad things are with me and how he just doesn't want to pay child support. 😑 When i found out he texted her saying there wont be anything between them. She then said "why did you tell her about us?" He then blocked and deleted her number. Apparently hasnt talked to her since according to phone records (on the same bill). I decided to forgive him if he changes everything. But if one more sign of anything pops up, ive built myself up financially and mentally that im out. As a woman , i don't understand how you can be okay with being with someone who has someone. And yes, i know he isnt shit either. I blame them both. Just wanted some non judgemental insight from anyone who has been here before. On either side, including the other woman.

Edit: He does not work there anymore. This all happened as he was transitioning out.