The right thing for him to do is to avoid the woman altogether, and to avoid women who flirt with him or try to gain his attention. It is so easy for the slippery slope towards infidelity to begin.
Almost everyone I meet with says some version of, "I thought it was safe, after all, we're both married and we weren't doing anything physical until. . ."
Once you've taken a few steps down the path the boundary lines keep changing, slipping, until you've gone further than you ever thought you would.
If he's like most men at this stage, he will strongly object and tell you, "she's just a friend, like a guy, and nothing more." Okay, maybe and maybe not at this stage, I certainly don't know and you don't "know" either.
I send you my best wishes to getting this resolved quickly in a way that supports your marriage.
