If it’s a cheap brake fluid, even water then system can be recovered but what if he poured engine oil or steering oil. because even if it’s a cheapest brake oil, it is suppose to be hygroscopic and should not throw these warnings. Driver must have poured engine oil in it.
Question: if that’s the scenario then can we wash the system using carburetor cleaner or injector cleaner. Later, drain everything using suction pump and put new dot 3 brake oil in it.
It’s his uncles fault, he should keep on trusting his drivers for fluids change.