Many of our ESL teachers asked us abt how to learn Chinese. If he is just curious about it I would recommend them to search for some Chinese class on Youtube. If he is ambitious, I would suggest him to take an offline Chinese class. Some of them got their HSK certification a few months later.
Learning Chinese always sounds like really hard work but actually it will be pretty interesting if you find the logic inside it. For example, "木" means wood. The word does look like wood. "林" means a small forest. "森" means a huge forest. You can see the logic of Chinese right?
So if you come to China teaching English, you could learn some Chinese. No need to speak like a native Chinese speaker but at least make your life easier in China. Most young Chinese could speak English but some times you will meet some old taxi driver or an old lady selling fruits. If you could speak some basic Chinese like "how much money is it?" "Could you drive me to the airport", you won't just make the communication easier but also will gain some praise.
The good news about learning Chinese is that the Chinese government has recently announced a new standardized Global Chinese Learning Platform. It’s under the guidance of The Chinese Ministry of Education, so it’s really seen as the gold-standard of learning content. Like all products produced by the Chinese government, the great thing about it is that it's using the latest AI technology but absolutely free.
You will find there're kinds of programs for you to learn Chinese. Learning Step by step or learning how to speak the sentence in common use immediately, you can decide it.
I do recommend our teacher to check this website. If you find it's too boring or too difficult for you, at least you didn’t waste any money on buying a class. If you got attracted, it's a good start for you to learn a new language. You already got a TEFL/TESOL certification, why not get an HSK certification! It will help you a lot when you apply for your next job.