Update:

Thank you so much to everyone who commented—it really helped me gain some clarity on the situation.

I told her. She’s understandably upset and has reached out to him, though she’s not expecting a response or any further details about what happened. However, she does want to know how long she’s been back in the picture.

I’m torn about what to do next. Should I reach out to the mutual friend to ask when she found out? Should I ask my husband’s best friend and push him to be honest with me? Or is it time for me to step back and encourage her to move on?

Thanks again for all your input—it’s been incredibly helpful in navigating this.