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.
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).