Which doesn't really matter in this case. The character for "he" in vernacular Cantonese is "佢", which isn't used at all in Mandarin.

The characters for "don't know how" in vernacular Cantonese are written as "唔識" in traditional Chinese or "唔识" in simplified Chinese, which are used in Mandarin, but never in this context (as the example given, the common way of saying "don't know how" in Mandarin is "不會" in traditional Chinese or "不会" in simplified Chinese").

In a sense, the relationship between various Chinese "dialects" and standard Mandarin Chinese is similar to the Scots language's (https://en.wikipedia.org/wiki/Scots_language) relationship to English.