I'm 35 years old and have been friends with this guy (let's call him Paul) for about 30 years. We grew up in the same neighborhood and stayed close throughout. He has been married to Megan for 5 years and together with her for 10. I know Megan very well too, the 3 of us have hung out often.
They had a baby a year ago. Paul has confided in me how stressful it's been for him at home since then and that he and Megan aren't getting along great. I have never been married nor a parent but I do understand that that's just part of becoming a parent and told him that pretty bluntly. I'm also okay being a sounding board for him if he wants.
A few days ago, he asks if he can come over to my apartment to hang out. He comes over at 10am and then an hour later I had to go to work. He asked if he could stay on since he'd had a bad fight with Megan. I said okay whatever.
I got done early with work and came home around 6pm to him FUCKING ANOTHER WOMAN in MY BED. I was shocked and so was he. He quickly tried to explain himself but I didn't want to hear him out. I literally yelled at him to get the fuck out which he did without even asking his partner if she was okay. I felt bad for her, made sure she was okay and then she left too. When I yelled, I made sure I mentioned and made reference to Megan and I could see his partners demeanor change. I'm pretty sure he told her he was single.
I haven't spoken to him since though he's texted me multiple times to check if "We're cool" and that I won't tell Megan. I'm so fucking grossed out and feel sick that he would treat his marriage that way, cheat on his poor wife and also betray my trust by lying to me and having sex with someone in my house without me knowing.
I feel like I need to tell Megan what's going on. It will cost me his friendship, but she needs to know.
TL;DR: My best friend cheated on his wife in my house. I want to tell his wife.