SusieK
Your Scottish brick wall may or may not come tumbling down (especially if your father doesn't test!), depending entirely upon who has or hasn't tested, and how far distant you are talking in generations (? your father's 2G or 3G grandparents).
Your mother's line, you are almost certain to get a good idea of who her great grandparents might have been, or at least which family lines they are associated with. She'd be looking for 3rd and 4th cousins, plenty of the latter should turn up, by excluding matches with her grandfather's line.
So, she may be disappointed not to find close matches, but should find out a little about her grandmother's family. She may strike lucky and achieve both outcomes - descendants of her great grandparents would be 2nd cousins of some degree.
Go for it, but be prepared to put some work in!
Regards Margaret