Update on partner sexting someone else

Update on my partner sexting someone. I've seen the screenshots of their messages and it went on for a while. Confronted him about it today (didn't mention that I've seen screenshots)and he said that I'm mistaken. She's been messaging him and he's not replied so she's telling lies about him because shes obsessed with him. Couldnt even tell me the truth 😤😤 then he accused me of ruining the relationship because I don't trust him!

He also said 'shes old so why would I want to?' Then I'd like to know why he was asking her to sit on his face 😭😂 for context she's 50 and we're in our 30s. We have kids and dreading co-parenting with him 💔