So a user came in with Safari not able to launch.
At first I thought perhaps the user account was corrupted, but I logged in as another user, and the issue persisted. Not launching and crashing. Just not launching at all. The dock icon didn’t even bounce.
Running
open /Applications/Safari.app
resulted in an error of LSOpenURLsWithRole() failed with error -10699 for the file /Applications/Safari.app
Because of System Integrity Protection, you can’t actually just copy the /Applications/Safari.app from another machine.
But it turns out reinstalling Safari works (the link in there is for El Capitan—I reinstalled it for Sierra).