He should have reported it as a mistaken kill.

How is he going to turn in his harvest card?

Did he tag it, how would you tag it and turn in your card if it were illegal and not reported?

If he didnt' tag it and not reporting it, that's 2 violations.

If he hunts again, that's 3 violations...

see how it esculates?

I'd recommend he report it and take his lumps.