His nut is not spinning. The head of the bolt is rounded.
Try to cut 6 new sides on the bolt like FF said. You donít have to make it perfect, just get it close. Then pound a cheap socket on it. (well not cheap, more like disposable)
If that doesnít work then you will have to cut the head off.
Once you have the head off and the K member dropped use a stud extractor or something like it to remove the bolt. vice grips probably wont do it.
Use can use pliers like these if you can find them, http://buy1.snapon.com/catalog/OBJECTS/19200/19159.JPG
I have broken off 7/16 exhaust studs with them before, not that you want to do that, but the do grasp a round object really well.
You may have to pull the lower half of the back mount off to drop the K member if you are just swinging it down. The hole will probably bind on whatís left of the bolt.