From When Scheduling Calls & You Poop Your Pants

What If I Do Answer the Phone on an Off Day?
A pilot’s obligation to accept a trip on X-days or regular line off days is
not absolute, whether the trip is the result of a White Slip (same day or
next day), Yellow Slip, Green Slip, or Inverse Assignment. A variety of
circumstances might make accepting such a trip impossible. For example:
-Not being physically located so as to be able to report.
-Having consumed alcohol.
-Lack of available child care.
-Lack of available transportation.
--Not being adequately rested. For example:A pilot, not on call, notifi ed late at night of an immediate report should consider if he is suffi ciently rested to accept such an
assignment.A pilot is not required to accept an inverse assignment that requires a commute that would prevent adequate rest for the
assigned trip.

Be polite but be insistent. If necessary, call your Chief Pilot.

I won't give more details but I'd like too but I think I'd give my buddy an ulcer.