I cannot convince someone to stop using charcoal lighter fluid in their fireplace.

I think this isn't right - could someone provide proof as to why this shouldn't be used in a fireplace?

He cannot convinced by common sense alone.

Mind you this man has a disabled daughter in his house.

Try looking up the MSDS data for it and see what it says

Why don't you think it's right? (I'm not saying it is right, I just wonder what you are worried about.)

There's nothing particularly wrong with that. Lighter fluid gives off very little vapor at room temperature, which is what would make using a liquid such as gasoline so dangerous. The lighter fluid burns on the wood in the same way lantern oil or wax burns on a wick. If it were dangerous to light fires with lighter fluid it would not have lasted long on the market as charcoal starter. The danger would be if the bottle was kept near the fire where it could spill out on the floor.

Does he just use it to start the fire, or does he continue to squirt more on once the fire is lit?