it could be an honest mistake and the top has accidentally been left off causing the problem, i wouldnt walk away just because of that to be honest, that seems a dramatic reaction
feel the hose at the bottom of the radiator and at the top, they should be a similar temp, if not this would indicate an air lock, also near the bulk head feel the pipes that go into the heater matrix (this is popular for an air lock and have been known to need to be back flushed, can make a noise)
most important is to take the oil cap off and check for mayo (this would indicate a costly head gasket).........then walk away
check the colour of the coolant, if its clear, it has been topped up regularly indicating a problem
did you notice if the heater was hot or cold ?