You report it could have been Sally or Billy

That isn’t how this works. Based on OPs description, the fact that Sally made a change is gone since the last editor was Billy. It looks like Billy made the change to both column 1 and 2.

You tell your boss:

"Just so you know, we only take a snapshot every X minutes... so there's a chance Billy wasn't the person who made that change. If you had told me you were planning to fire people as a result of this tracking system I might have spent more than five minutes hacking it together with a cronjob and a git repository."

Your boss let changes go through the production DB directly

Your boss decides to fire people as a solution

Your boss should be fired

I’m assuming the change Sally made was an unethical or illegal change.

Your boss doesn't micro manage

You should be taking care of this, not Billy

Firing a team member for a team mistake is micromanagement.

I’m assuming Billy was fired because Sally made an unethical or illegal change.

Billy did nothing wrong!