Just to be clear, her uncle has now lived in HK for several decades; I'm sure there was a several-month learning curve being able to understand Cantonese. It's not like Hindi and Urdu, where (I'm told) people with no previous exposure to the other language can communicate pretty readily.

But there are a huge number of words which are simply pronounced a bit differently; and the differences in pronunciation have a lot of predictable patterns. So once you're familiar with these patterns, suddenly a Mandarin speaker can understand a huge amount of Cantonese when spoken in context.