Here's the list of folks to call /things to do as I've heard them in interviews:

System/Domicile Chief Pilot
Captain's Friends (other captains) to encourage him to call sick
Professional Standards
Company Safety Manager / Director of Safety
Crew Scheduling
Drug / Alcohol Manager
Call in sick yourself
Offer to call in sick with the CA (bad food the night before perhaps?)

There were some other creative answers, but I can't remember them all. The big thing is don't let the Captain leave the hotel. But some interviewers might think higher/lower of you depending on their personal beliefs and experiences.

(For instance, I was more impressed by the people that not only protected the company, but also genuinely sought to get the CA help and not end his career by flat out calling the CP as the immediate first step).