If the key sheared and the balance and pulley moved from where they are supposed to be fixed to the crankshaft while the car was running, it would almost certainly have bent a few valves at the very least.
At worst, there could be quite a bit of severe damage including bent rods, cracked/damaged pistons, etc.
Obviously you can't turn it right now to find out if it moves freely.

Also, if the crank bolt came loose, You can be pretty sure that it had been worked on recently, probably by someone not skilled in good shop practices (proper torqueing) and not very thorough.

If you're looking at this car to buy, I'd be VERY skeptical.