Pardon? If you are a resident in the US, you pay taxes (whether you are a citizen or not).
If they're not in the US though (as a Canadian), they're not a resident, and therefore not a "foreign resident".
The entire situation is weird because as we both mentioned elsewhere, the correct approach here is a PEO (or setting up the appropriate hiring organization in Canada), rather than claiming to live (or work) in the US in order to get a TN (either or which would subject them to US taxation), and then claim to not when it comes to taxes.
What they can do is either a) have the US employer set up a Canadian subsidiary that handles their payroll or b) set up their own Canadian legal entity (corporation, sole trader??) and execute a services contract between the two companies. Then they can pay themselves under Canadian rules out of that company, or do whatever plumbers do in Canada -- sole trader or whatnot.
ianal, nor Canadian.