Hang on. I'm not sure if those two examples actually represent differences between Mandarin and Cantonese. Had you use the same grammar in Cantonese: 佢會說廣東話嗎？

Hang on. I'm not sure if those two examples actually represent differences between Mandarin and Cantonese. Had you use the same grammar in Cantonese: 佢會說廣東話嗎？ and simply replaces the words, it would still be comprehensible to a Cantonese speaker. It would sound a bit odd but that can be attributed to differences in colloquialism, sort of like how certain phrases are different in the various sub-dialects of American English. Also, I don't think the Cantonese example you used really means if he is capable of speaking Cantonese, which is what the Mandarin example say. The Cantonese example asks if he speaks Cantonese or not. The difference is subtle but still exists.