Good Morning,
I am sorry to hear of the problems your brother-in-law and sister-in-law are going through.
As your question is, "My main question here is should I completely cut my co-worker off. Or should I hear what she has to say?"
Here is what I would do but what you decided to do will be up to you.
I would stay out of it as all of this is really between the 3 people involved in the situation. You do not know the facts as you were not part of any of the affair and what you are using to base your feelings and opinions on is only heresay.
It feels to me like you may be overreacting to the drama going on around you. I would not judge your friend but perhaps be there for her as she did nothing to hurt you. She really does not owe anyone a reason for why she did what she did. That is her poor judgement and she has to deal with.
Looks like your cheating brother-in-law is getting help and trying to work things out with his wife while your friend takes the fall.
Years ago when I got divorced from my 1st husband everyone turned against me. All my neighbors who had been close friends, all of my ex-husband's family and the only support I had was from my Dad and Mom. It was very painful being judged and outcast.
Again this is just how I would react but I am me and you may see the whole thing in a completely different way. I am not judging you or anyone.
I hope you can work through your feelings of anger toward your friend.
Kindly,
Kitt