Assuming they were a close friend then in the most human way possible. If I hated them for some reason....same thing. A bullet to the brain would be a good one but I would do my best to not let them see it coming. Something like:
Rodney was bitten and we all knew it was a very short amount of time before he turned. He sat on the end of his bed and held his head in his hands.
"How could I let this happen?" he cried through his palms.
"I dunno man. But you still have a little time left. Lets make the most of it. Hey remember that time we met that chick with the face tattoo? I'm not sure who was more dangerous, her or the zeds."
"Yeah I remember her." he almost smiled as he looked up at me standing there next to him.
I raised my hand and offered him the last diet coke. I hated the stuff but he drank it ok. I popped the top on a Coke and held it up to make a toast.
"Here's to crazy women. Let's hope there are more to find."
He raised his bottle took a long drink.
I watched as the smoke from my gun slowly rose and dissipated. His body fell back and the bottle fell to the floor. I picked it up and sat it on the footlocker he kept at his bed. It might have been a waste of a bullet but it was quick and clean. He never knew it was coming.
It was time to move on. They already knew we were in here. I covered his body then picked up my pack and bedroll. Yeah time to move on.
Someone i didn't like.............
If we were in a secure location ( i know he just go bit so how secure can it be) but bare with me... If say he got bitten while out scavenging got bit and returned to the location... I would tie him/up like they did with that dude in 28 days later ... tie em to the wall let him turn and keep him... maybe for experimental purposes maybe just for the sick fun of seeing this A hole suffer the undeath...
If it was someone i loved... i'd give them a massive insulin shot (insuline is just something i would definately loot from a pharmacy upon outbreak.. partly cos my dad is diabetic and is my partner in crime always and partly cos I know how to us it well) anyhoo i'd tell them they wont feel a thing let them say their goodbyes and then give them a massive dose.. this would make them pass out within a few minutes fall into a coma as their sugar levels dropped.... then once they're out i'd use whatever i had to finish them off... gun if im lucky enough to have one a spade works just as well to separate their heads from their body..
In a story I am trying to get published I had a friend get bitten while on the way back to the base. The hero of the story shot his friend first then slowly unloaded the gun on the zombie taking out the limbs and finally the brain after it was pretty much "helpless". He took his time with the zombie out of revenge for his friend. In my first example I assumed they had made it back to base already.
I do like the letting them choose idea too but it is not always possible. In alot of zombie movies I have seen once they are bitten the wound will not stop bleeding and they bleed out quickly.
(1st opinion) ... with a good clean and nice shot in the head and make it double-tap on the chest...
or (for 2nd opinion ) crushing their heads off with hard objects (if there is no guns at all !)
cause somebody gotta do this .... or else ....
well Set it up with your friends\ the group of survivors your with before hand...ask em...hey puta if you get infected what do I do with you? me personally I just want a loved one to kiss the center of my forehead and then replace their lips with a gun and blow me away...maybe if I have time get me drunk one last time. and if you don't know una rameras then just kill them in the quickest and easiest manner.