Looking to freelance on a spouse visa – Advice on process

My wife is living in Japan with me on a spouse visa. She has a TEFL qualification and speaks basic Japanese.

A friend of ours has asked if she can tutor their child in English.

What does my wife need to do to become a freelance teacher and to legally be paid for this work?

I cannot see anything directly on this topic in the FAQ, and this seems like the most sensible subreddit for us to ask in.

Thank you for any advice you can give!