Make sure to follow it up with Robin Pearson’s History of Byzantium. He’s still centuries away from done, but I like it even better than Mike Duncan’s after it gets going.

You’re welcome!! Hope it serves you and your cousin well :)

Carl Humpfries’s Piano Handbook and Piano Improvisation Handbook are great, and cover enough for even an absolute beginner. I like noodling around with no previous musical knowledge, and they work very well for that. I think both include pretty decent sections on rhythyms, and discuss pretty varied styles.