Seems like there was a problem already, if he had to put any coolant in there in the first place.

 

It might cause a problem. Coolant is due to be replaced every 2-3 years anyway so I would have it changed to the proper MB brand or the approved Zerex.

 

We, members of this forum buy only materials with MB logo on it. That include gasoline, paper towels and toilet paper
MB coolant very likely is superior, but most of W123 run for 30 years on green stuff just fine.
If the coolant is old, I would change it per MM suggestion, but would not make emergency out of it.

 

The cooling system is a SEALED system. There should be no need to add any coolant. If you are adding coolant, there is a leak in the system that must be fixed ASAP.

 

Cooling system is not a sealed system. It is pressurized system. In extreme situation it will release pressurized vapor, or suck the air in. That allows for minimal loss of fluid, but usually not something that would require adding more often than once a year, or two.