dnd 5e – How can I help / encourage a magician with his accounting?

Use spell cards.

The main accounting problems with Wizards are knowing what your spells do, knowing what spells you have prepared and knowing what spells you know.

Physical cards with the spells written on them solve all three problems. You can buy them, or you can just make yours.

In this way, you always have the text of the spell at hand, and your player can make a deck of the spells he prepared each day. When they cast it, they can put it aside, and thus keep an easy record of the spells they have prepared. Physically shuffling the cards helps a lot to keep the spells in order, and it's much easier than writing and erasing a lot.

One of my fellow players uses this to play his magician character, and it seems to work very well for her: she keeps the cards in a large folder and exchanges them when appropriate.