It's such a horrible position to be put in
All I can say is be there for her when she needs to talk. Don't openly bash her husband, even though what he did is unforgivable. She's thinking those exact things and doesn't need to hear them from others. With that said, don't say anything to make her feel like what she is feeling is wrong. Let her talk to you and vent. Look up some marriage counselors in the area and help her find one.