I'm not quite understanding the issue. Maybe because I am not the romantic type, or maybe it is because I am too logical for my own good. What I don't understand specifically is that you asked if she was an "easy girl", and yet, claimed your friend acted like a player. Which... seems a bit hypocritical.

If this happened so long ago, and you know that your relationship with you girlfriend is strong and honest (considering she told you this in confidence gives me the feeling that the relationship does have safe communication), why is it something you're obsessing over? I think it would be healthier to put that thinking into good use by TALKING to your girlfriend, and not at all shaming her for what she did (because it isn't shameful) but that you're glad she trusts you enough to come to you with such information. And then be open about the topic.