How do you get a shot at the Combine?

A college football player form the US is looking to get a shot at the CFL after graduating next season. What can he do? I've told him to make a highlight reel.

Absolutely make a highlight reel. Unless he meets the definition of a "non-import" (which primarily means you are Canadian born, but really refers more to where you played highschool football) you can't be invited to the CFL Combine.
However every team has free agent camps each year. I recommend your friend visits the websites of individual teams for times and dates. And send the reel to each team.
Then, work hard===particularly on your cardio. If you don't have endurance in the CFL, you can't play....