No, it's not your business and you need to stop immediately texting and talking to him. You're going to be blamed for his mess, lying and cheating. Women rarely put the blame where it belongs on the men.
My friend is engaged, but he's sexting and what not with another girl should I say something or not?
Stay away from that mess let the chips fall
Best you steer clear of that altogether. Let her find out on her own.(and she will) Otherwise you're just going to get dragged into it and you don't want that.
Yep you gotta stay out of it. Even though you are not the one involved with the sexting you will be the one she blames because you will be the easiest for her to put her anger towards.... Just cuz you got involved. Chances are she already knows anyways.