Canadian to CUN with ORD stopover
 

Hi all, my Canadian friend would like to know if it’s possible to fly from Toronto to ORD, enjoy a day or two stopover in Chicago, then continue on to Cancun from MDW. What documentation other than a valid Canadian passport is required? Is it necessary to “pre-clear” for travel to both U.S. and Mexico visits when leaving Canada and would it be possible to then board a different airline a day or two later as my companion from MDW to CUN? Other option would be to just meet in CUN, but we’d prefer to fly together the leg from Chicago using my companion pass. Thanks for any expert advice!

Yes.
None

Your friend will clear US immigration and customs inspection at the US CBP pre-clearance facility at the Toronto airport. He will be in the US as a Canadian tourist. Nothing prevents him from taking any airline that he wishes to fly from Chicago Midway, or any other US airport, to Cancun.

Most Canadian passport users are admissible into the US without a visa, and so it’s generally not a passport control/immigration/customs problem to do what you have in mind with a short visit to Chicago with just the Canadian passport before continuing on a separately ticketed booking to Mexico.

Which Toronto airport is being used to fly to Chicsgo?

YYZ/Pearson has US CBP Preclearance, but as far as I know YTZ/City Centre airport does not yet (even as it was in the plans for 2023). You should probably consider sending the person their onward international travel bookings just In case it is asked for at YYZ in Canada or in CHI.