I think the point is that, from the original description, the $500K is not a "gift" as such. It's a loan and she expects the money back. This is unlike the money I transferred to my wife which I never expect back, it is a genuine gift.
Given that, I still maintain that it would be wise for the OP to consult with a financial advisor rather than get a very nasty surprise from either IRD or the Canadian tax authorities months (or years) later that he or his mother owes maybe tens of thousands of dollars. $300 spent on a consult now could save a very large bill later.