What would make a woman's legs get so weak overnight?

Better get to a doctor right away. Could be transverse myelitis, reactive arthritis or some type of spinal problem.

Is her mental capacity changing at all also? Like, used to be real good at remembering things, but now can't? Because that's a sign of mad-cow.

But that's really rare so it probably isn't that. Go to a doctor, a doc should be able to diagnose the problem.

Could be any number of things. She ought to see a Doc.

If she's taking any supliments she should stop.