First of all, you should learn the basics so that you can catch it in a better way. Learn all the alphabets read some books with the help of someone who knows Chinese language well.
Whenever you listen to him and he makes the mistake, correct it at the same time and tell him to use the correct pronunciation or you can ask him to use some app where he can listen to the correct pronunciation of the word by typing on it.
It is fine if you are charging a fee from your friend for teaching him and it's also good for your friend because you can teach him more perfectly than any other tutor because you both are friends and you help him in every step. You should ask a normal fee as you both are friends.
You can teach English using online tutoring platforms where you can find the tutoring jobs easily. You also get the opportunity to choose whether you want to teach as an online tutor or a private tutor, you can also choose both of them.