Hopefully this is just an over reacting weekend person shouting the sky is falling.
Although not ideal, if that's all they found, I wouldn't think most carriers would terminate over this.
I can see a stern talking to, an extended probationary period, etc.,
It would be swayed by the driver's past record and productivity.
It's a mistake the driver I'm sure will never make again.
And everyone should be thankful that it was discovered during an inspection and not during an accident investigation.
Good Luck to you and the driver.
My only advice, as it will be discussed at length, is tell the truth. If rookies, and there's anything not understood, get clear answers/proper training immediately.