A woman who marries a dangerous mentally ill man has a substantial lack of judgment. This woman is hurt and angry because her friends are not accepting the poor choices she has made. You can always reach out to her. Going out to lunch with her once in a while may be a way to keep in touch with her without bringing him into the picture. That said, I will say that one has little influence these days upon family members and even less upon friends.
Friendship requires things in common and respect and affection.
Commitment can make it last longer. If you are committed to give this woman a lifeline through occasional lunches you may be able to keep the crazy husband out of your life. From here I can't tell how safe that would be. You need to evaluate the safety of continuing your relationship with her. Dear Mrs. Web is not expert or professional opinion, but rather the opinion of one person.