If it were me I'd shovel out under the rear bumper and get the area behind the truck as level as possible. Take a jack and set it on a flat piece of wood or something to give it some "float" in the snow and attempt to raise the left side high enough to get some decent planks of wood, or anything that will fill the hole and provide more traction than the snow. Also let air out of the back left tire to cause it to squish a little and have a better "footprint". Straighten the wheels and come straight back.

One thing to remember about putting things under a tire for better traction, if the tires do spin they will try to shoot stuff forward if you're in reverse, so keep the drivers side door closed and don't let anyone stand around that area while trying to get it out. If you think going foward is a better option than that may work too, or rocking back and forth to try and get out of the hole that he dug.

It looks like he slid to the far side of the driveway backing in and once he was stuck he tried to pull forward to the right causing it to dig in more. In snow once the tire starts spinning you're done and might as well reposition and try to get some traction.

Good luck!