Hey candy_hearts. You know it's a mess, and you've have mucked up. It's difficult to put myself in your situation, but if I was you, I would leave well alone with this girl. If you poke at it, it could unravel even further. Is it worth asking hubby to delete her as a friend on FB if he doesn't know her - after all, if he doesn't, why would he accept her friend request?
Yes, you had an affair, but her bloke was the one that did the dirty on her in the middle of a 13 year relationship. If you didn't know about her, you're not to blame on that front - the fact you had an extra-marital affair is a whole other issue. HE's to blame for cheating on her with you. Her issue should be with him, not you. There are so many issues going on here - might help to deal with each separate one one at a time. Sorry - no further advice to offer. Just try and behave in future!