I always hesitate giving advice since I'm bound to "put my foot in my mouth" as well. With that said - I want to offer one take that has not been said yet. It looks like this issue is already a legal issue. Your friend and her husband may already have hired a lawyer or are figuring they are in legal trouble - hence her stance on not wanting to talk about it. Anything she says to you may have to come up in court - if it comes to that. Just as anything you say to her can be used in court - if it comes to that. Even if you don't want to testify in a court hearing saying things against her or her husband - you can still be subpoenaed. So, you may be able to offer simple moral support but with the legal issues hanging out there - I wouldn't expect her to want to confide in you.
I could be wrong - I often am. I just figured I would point this out.