dnd 5e – How to help / encourage a pitcher with his accounting

One of my players is playing his first pitcher in 5e and chose an assistant.

Unbeknownst to them, the class of magicians comes with a good amount of books that are kept in long breaks and downtime and this does not appeal to them.

I would like to avoid allowing them to change their character because we have had some problems with what we call "prostitution of characters", where people would try to seem disgusted with a character so they can interpret the new and brilliant idea of ​​character that has occurred to them. I found that allowing this greatly took away the RP side of things.

The player seems to find anything that is not combat that is not interesting and is often on his phone, while the others are playing RPing or resting / inactive.

Simply, what proven techniques work for a player to reserve more? I'm willing to teach them the rules, but they do not seem interested.