55-250mm lens on a 5D Mark ii?

To elaborate on why you can't adapt it: EF-S lenses mount with the rear lens element significantly closer to the sensor than EF lenses. Even if you were able to adapt the EF-S mount to EF, the first time you took a picture, the mirror would physically strike the rear lens element when it flips up for the shot, and would ruin your camera.

The 55-250 is a very popular lens; you could probably sell it for a decent amount.