Tell or not to Tell?

A few years ago, I had a one night stand with a guy from my gym. I’m comfortable with that. (Turns out he lost his virginity to me that night)
Today, I live in another state, and am VERY close friends with a girl who went to that gym at that time as well. She also no longer attends the old gym.

We discuss relationships often, and I eventually pieced together that a very formative relationship she had in her life was with my one night stand guy. She was still dating him when he and I hooked up that drunken night. Apparently, she was at the same party I was that night (I didn’t know her at the time) – he asked her to go home with him, she declined, then he asked me, and I said yes.

Her story about him is that they were pure, in love, both very religious, cared about each other, had an intense emotional relationship, and parted ways when she moved states. But she references him often, and has a very sweet view of their relationship. She doesn’t know that I had a meaningless fling with him years before I knew her. I feel crushing guilt that i haven’t revealed this to her – she references him often as we talk about our life and past relationships and our views on life, religion, our development, love etc. Every time she brings him up, I feel like I am being a bad friend by not disclosing this info to her. I predict that by telling her, I will relieve my “guilt” and while I don’t control her, based on what I know of her it is likely that it will spoil her view of their relationship together, that she will feel like she shared personal stories with me and that I lied by omission in NOT telling her I knew the guy, and that our friendship will be ruined. Neither of us desire to see this man again, we’re both dating others… I recognize that I am not obligated to disclose anything to her about my sexual history, but I have a nagging feeling of dishonesty. Also, worry that i may be sharing HIS story by way of talking about what happened, maybe it’s not my place to say?

C: Friend discloses personal stories to me about an ex, who I unknowingly slept with at the time.
T: I SHOULD tell her everything in order to be a good friend
F: Guilt
A: Ruminate on what to do
R: Eaten up by guilt, don’t tell her anything, get uncomfortable when she brings him up, feel like a bad friend..

It’s not really in keeping with our friendship to play the, “Can we just not talk about that particular guy from your past?” card….