sorry to see that happened. 100% of the time the keyway goes, its bad install. If he put it on with a impact gun without proper torque, that would definately cause the bolt to back out and sheer the keyway. I have seen 7-9 of these before, with the same results. I have yet to see a "weak" keyway before. Like posted above, follow the correct install procedure and you won't have any problems.

Torque properly and add that extra 90 degrees to lock it down. They are stretch bolts, one time use only!

hopefully you can just replace the keyway and the crack is ok!

wishing you the best, if you need anything let me know!