I know of at least one other instance. But note that the same is true of English speakers. Becoming literate in English isn't just a matter of learning the alphabet (otherwise a Frenchman automatically qualifies as literate in English simply because he knows the English alphabet). The way literacy is typically assessed is by asking you to read a typical paragraph from say a newspaper and then testing you for reading comprehension. Someone whose native tongue is English, but who for whatever reason has never had the primary/secondary education experience typical in an English speaking country is going to encounter the same sort of difficulty becoming literate in English as you friend in becoming literate in Chinese.

By his own account, it took this man [1] 7 years (during his late 40s/early 50s) achieving English literacy, and that happened in a English speaking country where he encounters English texts on a daily basis (I assume your friend lives & works in the West).

[1] https://www.bbc.com/news/stories-43700153.amp