There is a restriction somewhere. check your main shutoff at the meter and the service entry.
I think so too, and perhaps he can figure out where to some extent?
Thought experiment. Imagine 20' of pipe with a valve on one end and a restriction consisting of a pin hole on the other end, where the water comes in. At equilibrium the pressure on both sides of the pipe will be the same. When the valve is opened that 20' of water will come out, quickly at first, and then quickly taper off. If the valve is left open it will reach a different equilibrium, where the rate at which water comes out is just the flow rate for 80 PSI (or whatever the pressure is in the house) through the small orifice.
Now imagine that 20' pipe has a tee in it with another valve. Attach the water pressure meter there and open the valve. It will come up to house pressure. Open the other valve and the water pressure will fall to zero very quickly. (For some meters a reset button might need to be pushed to get a new measurement.) This suggests a way to find the restriction. Start at the hose bib were the water enters the house. If the pressure there doesn't vary (as in collapse) when a toilet is flushed then the restriction is inside the house, that is, downstream of the valve. If it does vary then it is upstream of that valve.
If the OP knows where all the pipes run in the house, and buys an adapter for the pressure meter so that it can go on a sink, or on a shutoff valve for a faucet or toilet, they should be able to determine roughly where the restriction is.
Is this house old with galvanized pipes? Does the OP find the aerators jamming up with tiny rust particles? This sort of restriction can happen when the pipes build up rust all the way across, or probably more commonly, when loose rust builds up into a sort of dam, with water only slowly filtering through. This can happen suddenly if somebody has been pounding on the old pipes, like to unscrew a stuck fitting. (Guess how I know...) If this is the case, and the problem is inside the house somewhere, it might be possible to clear the restriction (probably temporarily) by back flushing the pipe from downstream of the restriction. In the simplest implementation of this turn off the water at the curb, screw off the hose bib near the inlet (else it will probably jam with rust at its valve, or remove the valve from the bib), and run a hose from the neighbor's house to the bib the wife used to trigger the problem. (Flush that hose thoroughly before attaching it between the houses.) Turn on the water at the neighbor' hose bib, slowly. If the blockage is just a pile of rust it should be displaced and flow out where the hose bib was. If it is a lot of rust the water may need to be pulsed a bit to get it moving, and some wire may be needed to clear rust out of the pipe right at the exit.
If that works I would suggest then flushing it in the other direction. Remove the hose and either the valve or the whole bib on that end. Put the front bib back on or back together. Turn on the water at the curb. If the problem is solved some rust may come out, but the water should keep running out the back hose bib unimpeded. It is possible that the OP may need to go back and forth like that several times to work all the rust out of the pipe.
If ancient galvanized is the problem, repiping will be the ultimate solution.