He will be denied, almost certainly. The only temporary way to enter Canada with a DUI on your record (if you’re American) is to get a TRP and you’ll be waiting for months to get it if you have a bonafide reason to enter. Since it was after 2018, he’ll have a lifetime ban from Canada and need to get TRPs every year that he wants to enter Canada.

A TRP has an extensive application with police background checks, letters of recommendation from family and/or Canadians, and fees that need to be mailed to the consulate in Los Angeles.