D..
I hope you WILL continue to be close to her. She will feel so abandoned if you drop her.
What you should do is tell her that with what is going on with her marriage, that the only thing that will change is that you two won't talk about her husband. That subject will always be left out. You will not talk about her to him, or him to her. And they won't talk about each other to you. That way everyone can be comfortable, and you won't feel that you are put in the middle.
I think that this is the best way for things to work for all of you.
Good luck,
Dawn