Sexting with my best friend’s husband

My best friend’s husband is frequently getting caught on dating apps and talking to women on Reddit. They’re in an open relationship, but it’s one sided (she’s allowed to fuck other people, but he can’t). So my best friend gets very upset when she catches him and asked me to use my burner account on the app he frequents to see if he pops up.

I ended up finding him and I planned on flirting a little for evidence, but it has spiraled into full on sexting. He doesn’t know who I am, but has sent me pics and vids of himself. It’s so hot, but I feel terrible because I haven’t said anything to my friend. Don’t know if I ever will tell her at this point.