hi, without knowing the make of the car its hard to say if the thermosts can be accessed from the top or not, some are pretty difficult to get at and are also low down so that ststement may not be really true like your friends have said.
on the other hand, if it was me, firstly i would of filled the cooling system, let the car run, check the colling fan came on, let cool down and check level again, leave car parked on dry spot to check for leaks, faulty water pumps will always leak from a weep hole on the side of a pump, leaving evidence on the ground, even sometimes there is a rusty appearanve in and around the pump.
i think it was a bad call to remove the pump first off personally,,