Not familiar with the deer/road kill laws but the cop was going to ticket him for no tag? That's messed up.
When you hit a game animal that you want to take home and eat you have to call the cops.
Cops come out, ensure the animal is dead (or kill it if wounded/suffering), they tag it and your legal.
Cop would not have written the dude a ticket for no tag unless he was a dick. In most cases (I have done this), call the cops after i got home with the deer in the truck. They come out, tag the deer, write a report and go on their way. Most places in my area wont process a deer without a kill tag on it so unless you butcher yourself, you dont have a choice.