I'm sure we've all done things in the past that we're not proud of, out of pure self-preservation. Fear of the consequences of not fitting in, or in extreme cases, of being outed, is a very powerful motivator for playing along or staying silent. Imagine what would have happened 20 years ago if you'd spoken up. And realistically, would you have moved the needle on the prevailing mindset?
I know those fears kept me laughing at transphobic or homophobic jokes, or later just staying quiet. I'm occasionally ashamed of that, but forgive myself since one, I was scared to death at the time, and two, it was in the past, and I think just by openly living my life I've changed a lot of minds.
What you went through was exponentially higher in degree, but the same in substance as some of the things I've done. If you don't think dredging up the past would hurt the woman involved, I see no reason to avoid unburdening yourself. I did a little of that with an old coworker on my Michigan road trip, and it was good for both of us. On the other hand, maybe she would rather leave the past be. Maybe you can get a feel for her thoughts from her Facebook page.
Whatever you decide, please stop beating yourself up about it. You are wiser and better than you were way back then.
Hugs,
Stephanie