That's kinda interesting that he can do that. With a bit of recent Mandarin schooling I can see the langauges are quite similar, but as a kid whenever we had a Mandarin speaker visiting I was baffled. Same thing happened when I learned German, as soon as I learned the initial things like the pronouns everything clicked.
It's probably the case that all the often used words that you use in everyday life have changed, but once you get over the barrier there's a pretty smooth learning zone.
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.
It's probably the case that all the often used words that you use in everyday life have changed, but once you get over the barrier there's a pretty smooth learning zone.