create a global variable
chopper_boarded in the init file, set to false. set up a trigger, condition
player in chopper_name, on activation
chopper_boarded = true. then set up your ending trigger, condition
this and chopper_boarded and not (player in chopper_name).so, the player gets into the chopper, the variable switches to say he's in. the end trigger should be covering the ending helipad and will fire only when the player is inside, when the chopper has been boarded and the player is not in the chopper.
that said, if the player boards the chopper, then immediately gets out and runs into the trigger on foot, it will still fire... you sort of have to assume the player will not misbehave.