This is a little different from the usual 10% myth. I don't know of any research that says what percentage of the brain is in use at any moment; it would vary of course depending on what you were doing. And what counts as "brain"? Do we count all the glial cells (support tissue, non active) in the denominator? Ask her for a reference. If she says "It's generally known" be sure she's just retailing a myth.