That, I'm unsure of. Ross and Holly separated a while back, supposedly very amicably. I don't know if the affair started when she and Ross were still together, and if/when Ross found out about the whole thing. It seems like he found out the same time everyone did, when Jared's wife outed him, but who knows.
However, Jared was most definitely married, still is (for now). The three parties involved were all friends, so there's no way Holly didn't know that. So both Holly and Jared are assholes in this scenario.