Ask CPAxxx if they are declaring an emergency. If not then treat them as you would normal traffic. If CX want to put passengers on instead of fuel that is their problem, not yours.
Of all the long haul flights (god forbid you be fuel crit on short haul) that I have operated in CX I can honestly say that less than 0.1% have burnt more than CFP cont when I find myself approaching the HKG FIR