Every AC.com page has the country version flag at the top. The currency is put in bold letters at the top when selecting the fares.
The price at the end has the currency explicitly stated. Then you see it again after you put pax info in. Both times in bold letters on the same line as the dollar amount.
If for any reason the website doesn't know where you are, it will ask you on a big splash screen.
The website could misconstrue where you are for any number of user-related reasons. A common one might be the user is using a VPN.
I'm not seeing a way the airline is at fault here. Could it put up USD numbers by accident when everything says CAD to the user?