There are families where it's fine to drop in without warning and where it actually does happen, but it isn't okay for that to continue even if that's her husband's family culture if LW isn't comfortable with it!

It's her husband's responsibility to handle getting rid of his mother in that case (unless the LW is willing to just do it herself, which is less likely). I don't understand where he is in this story, and it makes a difference whether he's clueless, absent, or aware but making it her problem.