It would be legal... however, not sure of Canadian's flight rules, it may not have been necessary if their rules are the same as the U.S. In the U.S., a medical emergency would have prompted the controller to handle the aircraft as an emergency anyhow. The pilot may have declared 'mayday' because the passenger's situation may have gotten worse.
As for the cabin crew, it's probably up to them but more than likely someone probably assisted with the passenger so they didn't get injured any more during the landing.
Here in the US, of course, declaring 'mayday' means you're allowed to deviate from normal Part 91 procedures. If this would have happened in the US, I don't think the mayday call would have changed anything since the aircraft would have likely not have deviated from any of the rules.