A 50mm lens will have a shalower DOF on a 5D than on a crop senzor camera because:
Firstly, DOF is endeed determined by 3 things, and these things are: Focal Lenght, Aperture and Distance, not senzor size, even though senzor size affects the distance from where you take your shot.
So a 50mm lens will have shalower dof ONLY when you compare it with the same shot size taken with the same lens and a crop senzor. Thats is because you will get closer to get the same shot size with a 50mm lens on 5D than with the same lens on a crop senzor. And being closer will affect the distance where you set your focus and that affects how shallow your DOF is.
Hope I'm right
