I'm trying to create an encounter where PCs are climbing a vine or rope down a 100ft shaft. About half way down, a creature flies up and attempts to grab one of the PCs to fly away with her.

The monster I'm using (a sabosan) has a fly speed of 40' (good) and the Flyby Attack feat (if that's relevant). My understanding of the rules is that:

 - For round 1: the sabosan just needs to fly to the PC and grapple her. 
 - For round 2: the monster must make a grapple check and, if successful, can move up to half its speed with the target. Because the PC would be in a hazardous location (50' in the air), the PC receives a +4 bonus on this check. And because the grapple check is a standard action, the monster could only move 20' with the target on each turn (if the grapple succeeds).

Questions:

 1. Does the above sound correct/fair for the rules of a flyby grapple, or is there something I'm missing? 
 2. Also, should/do I need to account for the PC holding onto the rope while climbing down? Perhaps give the PC a +2 to their original grapple check?