Car has been sitting since 1955...free breathing System, no idea how much Coolant has evaporated over time.
Might not be any way to reach existing Coolant with the Testing Device tube if things have evaporated down a ways.
If he adds Water or adds Prestone, there's no way to circulate either, to bring level up of exsting Solution, to be reached by the Tester, since Car is not running.
I can ask him to unscrew the Radiator Cap and look in with a Flashlight...and if Low, to just add Prestone, and hope for the best...but I just wanted to feel sure.
Probably there is a Radiator Drain Petcock on the inside Bottom of the Radiator, but I do not know...if there is, maybe he or someone he can have help, could open the Petcock, drain out some in to a Cup, then test that sample.
I remember some old Cars had a Radiator Drain Petcock and also a Petcock to drain the Block...if this Car has those, he could just drain the Coolant out, close Pet cocks, add Prestone with no circulation, and that would likely do...