Yes, absolutely. She will need your friendship.
I'm a big fan of forgiveness; it is almost always the right response. The reason, as I mentioned yesterday in another answer, is that none of us is perfect. We all make mistakes. When we do, we want others to forgive us. What if the tables were turned and you made a bad decision that hurt her? Wouldn't you hope she understood and eventually forgave you?
I know you're upset that she just dropped you, and I would be, too. But if you call yourself her friend, then you must do what friends do: forgive. I doubt it was her intention to hurt you; few people voluntarily cut ties with everyone. Did she ever give you a reason? My hunch is that her abusive boyfriend forced her into doing it because he knew you had his number. Perhaps you think that she should have stood up to him, but unless you've been in a similar situation, you don't know how you would react. He probably gave her an ultimatum, and she chose him over her friends, which happens more than we like to think it does, particularly in codependent abusive relationships.
Hopefully she'll be rid of this guy soon and come back to you, and I hope you will take her back. A best friend of ten years deserves a second chance, don't you think? I'm not saying you have to pretend it never happened. You and she will need to have a lot of conversations about what she did, why she did it, and how it made you feel. Forgiving someone becomes a bit easier after you have vented and heard her side of the story.
There is also a chance she will dump this guy and still not seek you out because she is too embarrassed or afraid to face you. You might end up having to forgive her without her participation, like when a parent or spouse or friend dies and we still have unfinished emotional business with them. It is harder to forgive someone who's not in front you admitting their mistake and asking for mercy, but then, you're not doing it for her so much as you are doing it for you. Forgiveness is more a gift to yourself than it is to the person who hurt you, because it lets you flush those toxic feelings and get on with your life. And that's another reason I'm a big fan.
Thanks for the question.
This happened to me twice, with two of my closest friends. They both took about 18 months to "reappear" after the break ups. One was much more humble, but the other married the idiot and divorced him in six months like it never happened. The one who admitted her mistakes really learned the lesson and really grew from the experience. We've never been closer.
Just forgive, and move forward in YOUR life. A true friend will always come back to you.
Please don't leave her hanging because you want to get back at her for leaving you. She needs a friend, a real one.
I had an experience like this and I was the one in the relationship. They are extremely hard to understand when you are in one. But when you finally get out you realize what was happening the whole time. As for my friend she totally gave me the ultimatum thing like me or him, and I chose him. But when we finally broke up I didn't go back to being best friends with her. I just think that a true friend will be there no matter what and not try and tell you what to do but to be there for you after you made the mistakes, to help pick you back up again.
I've been in your firend's situation. Not being there for her will make it harder for her to leave when she's ready. Part of the abuse is isolating someone and taking away their support system so they feel they can't leave.
Cary's right. I work in domestic violence services, and nearly all of my clients (formerly abused women who left their relationships) were isolated by their abusive partners, usually relatively early on in the relationship. Cutting off a woman from her loved ones is a method abusive partners use to gain control and power in the relationship: "If she can't depend on anyone else, she has to depend on me. I'll be all she has." If you don't forgive her and allow her back, she could have no one to turn to when she finally realizes this relationship is abusive. That will make it even harder to leave, and I hate to say this, but if she doesn't leave, who knows what can happen? She could die. 1/3 of female homicide victims are killed by a partner/boyfriend/husband. So. Forgive. It's not her, it's this crap relationship.