First off, she needs to be in counseling on her own. Not just marital counseling. She needs to see someone that is here JUST for her and with out her husband there. I think an anger management course would probably help a lot.
I have a friend who was raised in a smacking house (at times it was down right beating) - some of her sibs go WAY out of their way to avoid confrontation and others smash right into it head first. And we are always products of our environment so of course how she was raised has an effect on how she is now.
Some people are universally abusive and others have triggers. (ie, when her husband is not helpful and then smarts off about it).
My suspicion is that she may also come from an alcoholic family. She sounds like a lot of what I've seen as children who had to deal with alcoholic parents and then didn't learn how to really have healthy relationships. (It took me a while to learn how to interact with a significant other with honesty - and I'm still working on it)
I would recommend she get in touch with Al-Anon if this is the case. Even if she is dealing with issues from past drinking problems they can still help point her in the right direction.
And she needs to see a counselor that she feels is helping. I can't say enough for Dr Lichti in Hagerstown but that might be a bit of a drive for her.
The first step is realizing you need help.