I've been reading everyone's posts with interest. Dawn, I'm impressed by your balanced and pragmatic approach. If everyone could manage your level of honesty when things/people go wrong, the world would be a happier place
I feel confused by the amount of black-and-white thinking on your thread, not to mention vitriol and blaming. The thing happened. You can't change that, though I'm sure everyone would if they could. You have a healthy, happy marriage which weighs more in your life than an incident, which happened before your marriage started. You have a long-standing friend going through a turbulent life crisis. I feel the only thing you can do is clarify the facts in your own mind - which you have done: props to you and DH (more you, but he spoke up so him as well!)
In the general run of events, you would never have know about it and your friend would never have been troubled by it. I have some empathy for her. Perhaps people who've never 'needed' to rewrite history can't understand it, so here's my attempt at a potted version. My childhood is a blank to me, and so are numerous things I did in the years that followed. Each time I remember something from childhood, I get another recollection of a stupid thing I did in my twenties. I didn't consciously choose to forget: it was a childhood survival mechanism that, unsurprisingly, took a while to switch off. The facts that your friend has been a bit of a boundary-pusher, and has now had a breakdown, suggest she's in a similar boat.
I don't actually think you can do any harm by talking to her yourself. The worst that can happen is that nothing will change. You're a good friend, and a wise one by the sound of it, so perhaps you'll find the right combination of resolution and gentleness to let her unburden the issue without hurting anyone. That would be the best outcome, wouldn't it?
If it all looks like going pear-shaped at an early stage, then you can always try backing off then suggesting a joint session at Relate.
From your own point of view, you've already done the hardest part (getting the whole story from DH) so the main thing, now, is to come out of this without blaming, resenting, or shouldering anybody else's issues. I believe you'll do that just fine