The bottom line is the rules for such a purchase by a Canadian VISITING the USA have not changed.
All sales (private or business) must go through an FFL. The FFL will have the firearm shipped to Canada. The Canadian CANNOT take possession!
What did change is the 90 resident requirement for those making a home in the USA. 90 days is no longer a requirement. A purchase can be made almost immediately.
Nonimmigrant aliens lawfully admitted to the United States without a visa (e.g. Visa Waiver Program), will not be prohibited from shipping, transporting, receiving, or possessing firearms or ammunition, provided that they meet State of residency requirements and are not otherwise prohibited from shipping, transporting, receiving, or possessing firearms.Regulations for Nonimmigrant Alien
To acquire a firearm, you must demonstrate the intention of making a home in a particular State. The intention of making a home in a State must be demonstrated to a Federal firearms licensee by presenting valid government issued identification documents indicating an address in the state in which the licensee’s place of business is located. Such documents include, but are not limited to, driver’s licenses, voter registration, tax records, or vehicle registration.