What would you do?

My girlfriend (20F) and I (20M) have been together for about 6 month and things have been going quite well with some hickups. But there has been a problem and I’’ not sure how to deal with. Recently we were talking about out sexual history she told me that she went all the way with a guy after a couple of dates and their fling didnt amoubt to anything. She told me she regrets it. Then I found out that I know the person and many of my friends are good friends with him and I dont know what to do because I fear that people will know What the guy did with her how quickly and easily. For me that would be really embarrasing and humiliating. I also worry about How awkward and humiliating it will be when I’m in the same room with the guy because we sometimes hang around in the same group of friends. She told me it’s the past, and it was a mistake and it doesnt matter. I dont know what I should do with my relationship and how to get past this or can I. Can some of you give me advice how would you handle this and What would you?

TL;DR:My gf(20F) and me (20M) have been together for 6 months and things have been going quite well. Recently I found out that some of my friends are friends with a guy my gf was intimate with and they only met half a dozen times? For me this is embarrassing and I don’t know what to do? Nobody except me and my gf know about this. The previous post was deleted.