It's been my observation that a wayward spouse will usually admit to one level less than what really happened. It's highly likely she had sex with him, and my advice would be to proceed under that assumption.
Starsky
Spot on as always Starsky...I've been operating under that assumption since the 9th when I found the second round of dirty messages, and actually I'm guilty of the, "admission of one level less," behavior too. About a week after I met my W I went on a planned spring break trip with my roommate, his GF, and her sorority sister, with whom I shared a room/bed. Of course when I got back my future W asked if anything happened and I said we had kissed. Much later, after we were engaged I revealed that we had sex after heavy drinking. So I hear where you're coming from.
I know I'm getting trickle-truthed but it's a HUGE improvement over her attitude when I first got back a few weeks ago, which was disrespectful and full on denial even after I found the first set of pictures. She does now say that she wants to work on our marriage and that the last week has made her want to be closer to me, but the remaining contact with the OM is still preventing me from talking about the future with her. Like I said, things are better, but we still have work to do.