was at a single story residence today. When the toilet is flushed the pressure at all other fixtures drops. The pressure at a hose bib was 85.
What the is going on here? is this the sign of a failing PRV?
The stub outs for the water heater were copper so I'm thinking the house is plumbed in copper.

Somewhere the line is restricted. It could be a clogged prv, a partially closed valve, or any number of things. I went to a house once that the line pressure was 170 psi. The plumber before me installed a prv and the house lost all volume. The prv was working correctly, but the galvanized lines were almost corroded closed. I took a section of 1/2 inch apart at a union and looked inside with a flashlight and the inside barely had an eighth inch of pathway inside it for water flow. Just because there was copper in one spot doesn't mean the entire system was copper. Check it at the water meter, the main water valve, and make sure all the valves on the incoming lines are open all the way. If the main water shutoff has a compression stop, the washer could have come off and is wedged inside the line somewhere. Without a thorough inspection, any assumptions that you make are just that.

If you have a PRV, that is the classic sign that it is failing and needs replacement, (I and most plumbers do not waste our time trying to repair them).