Actually, HP speaks to her son mostly in Mandarin, but he is able to speak in snippets and pidgin Cantonese with his peers.
I did ask HP how she learnt Cantonese. She mentioned that she spoke a different dialect from Eastern side of China, and had married into a Cantonese speaking family in the South. Her in laws could only speak Cantonese, and no Mandarin at all.
Mandarin was not pushed as the main language in China until maybe forty years ago. Even News broadcasting in Guangzhou is in Cantonese, and not Mandarin like most parts of China. This created a special situation for that part of the country. Hence the main dialect was Cantonese with not much monetary incentive to learn Mandarin.
The younger generation perhaps are able to converse in Mandarin, but the older folks had no motivation to do so, as with the case of her in laws. To some degree, Guangzhou is like Hong Kong, with Cantonese being both a dialect and a language.