There are mothers out there who would still give their grown kids a great deal of grief over their dating choices. Her mom might be one of those women. If so, I feel for her, but it's still something she's going to have to deal with at some point or her mom will always be a problem in your relationship. I hope she really had no other choice besides moving in with her mom. Otherwise, the fact that she chose to do so says a great deal about how much effort she'll put toward standing up against her mom. If she won't protect her own children from their "Mean Grandma," then she won't protect you from the lady either.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy