You can not drive across the border to buy a car. That's considered non-essential. Rumors are they can fly in, but I don't know how true that is since I don't work at an airport. If they can fly in, they can export it with US CBP no problem (as long as they follow the procedures) and then it's up to CBSA to let them import the vehicle and/or even let it in. Best bet is to hire a commercial carrier or transport company to do it.