I would suggest your husband begin attending Al-anon meetings. It will help immensely.
If his mother is elderly and enduring abuse he can call Adult Services and Aging (part of the Department of Social Services) and report the abuse. They will investigate. I'm in the US. Not sure how other countries are set up.
You could provide his mother with the numbers to Domestic Violence Hotlines and offer her a ride there if she is willing to go talk to them.
I hope someone with more experience with domestic violence comes along to give you some good information about how you might support her in that area.