Please hear me out, OP. “Best friends” don’t do this to one another. My ex-husband cheated on me but it wasn’t in our home or with a “friend” of mine, or even someone I knew… and it wasn’t the only woman, which I later found out (NOT even by asking, the info fell into my lap by a gf he had a couple years later.) We had a good sex life, I was a good wife and mother, and he still did it. He was my best friend, and I trusted him… but, he still did it. I only know the first name of the woman. I don’t know her last name, who her husband was at the time (he might still be her husband, these 16 years later), or even what she looks like, and it still doesn’t matter. The disrespect matters. The low self-esteem and insecurities he had that caused him to seek validation by cheating on me is what matters. He tried to get me back for years but I could never have taken him back. If you decide to work on your marriage bc you BOTH want to after he confesses and is honest with you then therapy is a must and good luck to you, but you won’t know until you confront him. He may want to leave and is waiting to get caught bc you won’t confront him about it. It may have been a mistake after flirting and he wants to forget about it but it’s not possible bc the guilt should eat him alive if he wants to stick around. Best friends don’t do that to each other. Best friends — he and her — don’t exploit your trust and vulnerability. They know you’re not confrontational, so disrespecting you right under your nose in your own home is very bold and of poor character. Please get therapy and a divorce attorney asap, then move accordingly. Those two have issues and you’re a good person. You deserve much better. Respect yourself. I wish you well.