There have been a few times where I've allowed someone to throw a spare ball after a pin had been knocked over by the setter without replacing the pin, however my stipulation was that I would let it stand if they would have gotten the pin on the spare anyway or I would adjust the score accordingly if it was obvious they would not have gotten the pin.
This is a bad idea because it leave room for interpretation. Take the time to have the pin reset and do it right.
Actually last week there was a similar situation to this in that someone on the pair next to me left a 4-7 but had a slow messenger come over and knock out the 4 just before the setter came down. No one on their pair saw it so they called to have the 4 placed back on the lane and he would throw at the 2 pin leave instead just the 1 pin leave. When I told them that he had a messenger come over and knock the pin out they had a short discussion and all agreed he would keep the 8 pinfall as it was and go from there. No one cheated but they all talked it over and came to an agreement in a respectful manner.
There's really nothing for them to talk over. They don't get to decide what pinfall to use. If no one sees it, you do NOT set the pin back up. You go with 9. If someone sees the machine hit it then you stand it back up. There's no vote about it.