I'm curious, did you start to sense that something wasn't right before she started coming onto your husband? Were there other warning signs that she wasn't genuine, was attention seeking even to the point of it affecting her relationships with friends, or manipulative? It would stand to reason that someone that lacking in perspective (enough to risk your friendship and her marriage when you both have children involved) would have given you other clues there were deep seated issues even before that happened.
You were friends for a long time, 4 years...I think if she truly has a personality disorder you'd have seen many clues in that time. With my ex friend there were numerous warning signs that I ignored or made excuses about for a while, but it was like her mask was slipping more and more as time went on. The lies, deceit, manipulation, taking advantage of people, attention seeking, disloyalty to friends, husband, family...in the end it was all so overwhelming that I COULDN'T ignore or excuse it anymore.
If this was the first and only issue you had with this woman, even though its an extreme case of betrayal, it does make me wonder if its not an isolated incident in her life. Yes, stupid, immoral and lacking in judgment, and unforgivable if you ask me....but perhaps not a personality disorder.
It IS interesting how she managed to worm her way so thoroughly into your life though. That was my experience with my ex friend too, when we first met it was like a full on assault to woo me
If I hadn't been so vulnerable at that point in my life I'd have been a lot more wary.