Maybe it's just the mood I'm in today, but I'm finding a lot of these responses to be a little harsh.
While I do agree that the situation is not pretty (and one that I wouldn't be okay with, either), there's a lot of conclusion-jumping going on here.
For starters, it's apparently an automatic that this guy is a predator of some sort AND that she was taken advantage of. That MAY be the case, but hey - two adults can also have a consensual sexual relationship if they so choose. I don't get the impression from OP that his girlfriend feels like she was taken advantage of, or that she has any regrets (other than the strife this has apparently caused now).
Many people have had sexual relationships, or even marriages, with people who they, at some point, wish they could erase. I kick myself at my lack of judgement over my first marriage, as well as a past girlfriend or two. Hindsight is 20/20.
In this case, the person is in her social activity circle, which is unfortunate - but happens. This doesn't sound to me like a close group of friends, but rather a group of people who share a hobby/activity/passion.
There are accepted boundaries between the two, and they appear to be honoured. She also told OP about it in the first place, which is good. No big secret.
OP simply has to decide whether he can accept that she had a "fling" with somebody she's still in contact with, or not. It's that simple. It doesn't sound like the two of them spend any time with each other alone, or doing anything other than their mutual activity.
How HE feels about the matter is his prerogative, and I think he's justified either way - there's no wrong answer.
However, to make assumptions that she'll somehow be sucked back into this guys bed, or that he's a sexual predator, or that she's somehow "broken" or their fling never really ended, etc etc etc. is crazy talk, IMO. And it's also highly insulting to a woman none of us know.
A lot of you have decided this guy is gross (and he may be) and that she's a risk to have another fling with him, like he has some sort of spell over her, or that she's not capable of making good decisions. All because she had sex with him a few times. A lot of you have taken the information OP outlined and decided she's of low moral character simply by having a sexual relationship with a guy who's not portrayed in the best of light. Everybody's assuming HE bedded HER. That she was somehow coerced into it. He's being vilified for being a Lothario, and she's being vilified for being taken advantage of. And what's worse is there's an assumption that she'll "fall for it" again. How condescending is that?