WestJet will check for a passport at boarding for the first leg (since it’s an international journey) but probably not for a valid visa. A Canadian airline employee - working a domestic flight - wouldn’t qualified to determine that.

When connecting, the person will need to clear through US Customs at YYZ before going to the gate. That’s where they’ll be stopped if they’re not admissible to the US — by US Customs, not by the airline.

I’d expect they’d make it as far as Toronto with no problem, but they’ll be stopped there if they don’t have the proper visa to continue on the last leg.