@ Wendz,
If you were her best maid it means you are good friends. Well i think.
Just tell her in black and white, this issue of your mum is messing your marriage, period. And give her some of the views expressed especially by our brothers/fathers here

.
As a friend, tell her bluntly she has two choices-to listen to her mum or to listen to her hubby. But in my view, she will also need to:-
-sit her mum down and tell her that she wants to run her home herself (with respect, asitusi mama yake)
-apologize to her husband
-be ready to have a new start where she and Mr run the family as a unit.
-Pray like mad that Mr. will now listen to her.
I had a similar issue with a friend and i told her just that. So far, so good. It was hard for her to do it but once she did, even she saw the difference it made.
All the best, sometimes its really tough being a friend.
....above all, to stand.