What can I do if my collaborator is enthusiastic, but their PhD student is not?
Instead of "making" the student do or be anything--which is impossible anyway--you could try treating the student as a junior collaborator rather than a gopher. Or maybe the student simply doesn't want to do this project at all. 

Some people are passive-aggressive because of personality issues, but in the case where there is an enormous power disparity, many people are passive-aggressive because they don't have any other way of expressing their unhappiness about something. So it's up to you (or the advisor) to broach the subject.

You could take the point of view that the student should do whatever we want and shut up about it, but pragmatically, this approach hasn't been working out so well, so maybe it's time to try something else.