Just to update, the mechanic is a retired MB dealer tech with 40 years of experience. He works on a short list of cars for people that he knows. I'm fairly certain he put a used monovalve on the first time, one he had laying around. It was leaking and caused the overheat. He apologized for not test driving the car, fixed it, and dropped it off working fine.

The heat worked properly for about 2 weeks, the coolant temp would get to 80. Then the engine stopped warming up properly and was not producing heat. I drove the car myself, even with the block heater running prior to start up, after 30 minutes of driving the temperature needle was barely above 40. I made a grille block and the needle would get up to 60 and hold while driving, but there was still just cold air coming out of the vents.

The car has been back to the mechanic friend twice with no resolution, it's well over an hour drive each way, quite a trip to make with no heat for a 71 year old woman. Now he is sick and can't look at it again until further notice. My friend is concerned the mechanic might be suffering from the early stages of dementia, though I wonder that about her when she tries to relay what he told about the car and seems to have no idea. The mechanic did not return any of my calls so I'm in the dark.

I was going to try and not get in the middle of this debacle but it is getting a bit ridiculous.

I'm thinking the recent overheating took a toll on the thermostat and it is stuck open. So this is not a monovalve issue now, but the mechanic is still chasing that prior problem. Could be both though.