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

Use spell cards.

The main accounting issues at Wizards are knowing what your spells do, knowing what spells you have prepared, and knowing which spells you know.

The physical cards on which spells are written solve these three problems. You can buy them, or you can simply create yours.

In this way, you always have the spell text itself at hand and your player can create a spell deck that he has prepared each day. When they cast it, they can put it aside and keep an easy record of the spells they have prepared. Physically blending cards helps a lot in keeping spells straight, and it's a lot easier than writing and erasing a lot.

One of my fellow players uses this to play his wizarding character, and it seems to work very well for her – she keeps the cards in a big binder and swaps them at any time.