Assuming they don't conform to US safety standards (and aren't easily made to conform), your best bet is to aim for this provision:

> Nonresidents may import a vehicle duty-free for personal use up to (1) one year if the vehicle is imported in conjunction with the owner's arrival. Vehicles imported under this provision that do not conform to U.S. safety and emission standards must be exported within one year and may not be sold in the U.S. There is no exemption or extension of the export requirements.

There's certainly a question of if it's personal use if your canadian friend leaves their vehicle at your place and you drive it around. But your friend can certainly get it over the border and I don't know how much enforcement you'll get after that. You will want the vehicle to return to Canada before the year is up.