Wow. My gut reaction is if this is a close friend, loyalty to her comes first. It doesn't really matter how you found out about the stealing, but now you do know, and at some point she will know that you know. You don't have to tell her everything, but you can say that you've heard some of what's happened (but you haven't heard the dh's side of it), it sounds pretty serious, that she really needs to talk with her dh if she hasn't, and that you're there for her if she needs support. Then she can ask you for more details if she wants.
I can imagine that she suspects but doesn't want to admit it, is unhappy and maybe too ashamed to talk about it. I would be, if it were me. Knowing that you know some of it and are ok with it may be a big help. This sounds like gambling addiction, and she may need to know that not everyone thinks that her dh is evil and that she is dumb and naive.
But it depends on your relationship with her. You have to judge if her keeping her pride is more important, since you have a professional relationship with her too. Either way it will be uncomfortable. My sympathies!
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett