[David]

Based on what Chris herself has said about her musical training in the Keyboard magazine interview and elsewhere, her classical training wasn't very extensive. She abandoned it early on to start playing early rock and roll, like Fats Domino. From there, she played rock and blues, learning how to replicate the styles of Bob Hall and Sonny Thompson.

I'm going to assume that she can still read staff notation, but at an intermediate level. In that sense, you never truly forget everything you ever learned about notation, but you get better or worse over the years, depending on how much practice you're getting. (By the way, there's plenty of classical music that an intermediate reader can amuse himself with.)

I'll second the opinion already given that there isn't any Fleetwood Mac music that would have required the keyboardist (or any of the other guys) to be able to read staff notation, let alone read it fluently.