Not sure if this has been addressed elsewhere but I want to be totally clear on the process before I buy the AC Z class ticket for my client:

From AC's website, I already know my client (holding a PRC passport and US visa) can fly from PVG to BOS connecting at YYZ without needing a Canadian transit visa, but it's the other direction I want to clarify - can they fly BOS-YYZ-HKG with a layover at YYZ without needing a Canadian transit or entry visa and does the AC check-in staff at BOS know this?

We can't hold the ticket too long and it's the last Z seat - so if someone can offer advise on this quickly, it would be greatly appreciated.