At 10 a.m. I had a telephone caLL from Mr. Barros: No water! I coached him
twice through the simple routine of turning on the wall switch and triggering
the pressure control valve. Then success! 

My thought is that when he reports to you that he has completed the plastering
you should instruct him to drain the tank as follows:
1) Turn off the electricity to the pump with the wall switch above 
and to the left of the pressure tank.
2) Without loosening it, pull the garden hose which was used to provide water 
for the plastering from the first floor in to the basement.
3) Hold the nozzle of this hose over the sump in the basement, and open the
nozzle to let it squirt the pressurized water into the sump.
4) Remove the nozzle and insert the end of the hose into the sump to
complete the draining.
5) If Mr. Barros wishes to take his hose with him, ask him to insert the
end of our garden hose to which his garden hose has been attached into
the sump.   

Temperature forecast for Nantucket:

Dec 3, Tonight Partly cloudy, with a low around 36.
Dec 4, Tuesday Night Mostly clear, with a low around 26.
Dec 5, Wednesday Night Mostly cloudy, with a low around 28.
Dec 6, Thursday Night Mostly cloudy, with a low around 34.
Dec 7, Friday Night Mostly clear, with a low around 25.
Dec 8, Saturday Night Partly cloudy, with a low around 26.

I doubt,  but can't be sure, that the tank would freeze in the coming week. 
It's for you to decide whether to ask Mr. Barros to drain it in the evenings 
preceding the cold nights. A replacement tank would cost (only) $280, 
with another $50 for associated tubing pipes and valves.

==============

I looked at instructions for installing and maintaining pre-finished
flooring; and consider it a gimmick to raise manufacturer's profits,
which cannot be repaired or re-finished: a bad bet in an unheated
humid house in Madaket.