I agree with Jeffro above in that the past is the past. Also:

1. 13 partners really isn't that much. Seriously. I knew girls in college that would have that many in a week.
2. Choosing to participate in a couple of 3-ways doesn't necessarily make a person promiscuous, again going back to the only 13 partners total.
3. Considering the partners you've mentioned, it seems like she was a young person trying to find herself, not just a person sleeping around.

Really, the only concern I would have is when the number of partners increases, the chances of an STD go up as well and the 3-ways could have increased it. She might want to get checked just to be sure nothing bad happened there. Other than that, her choices aren't that questionable; drugs, having 20 guys pull a train on her, binge drinking ... those are questionable choices.

Overall though, I would say the only real issue is the one you are having concerning how to handle it. Don't dwell on the past; it's gone. Now is the only time to be concerned with.