Sadly, there is a lengthy "engine health assessment" TSB for the MB M272/273 engines for oil consumption and misfire complaints. Usually resulting in new cylinder heads.
However, if it were mine, I'd try some BG MOA treatment a few times on it first, then make certain it is filled with the proper 229.51 spec motor oil, 5w40 preferred, with a good filter (Mann, etc.).
Although if the cylinder is "dead", and it still has compression, then you may indeed have something else going on. You can do an output test on the injectors with a proper scan tool. You can also use a stethoscope on them, but if the problem with misfire is NOT the injector, the ECU will cut fuel to it anyway, so you have to clear the DTC, start the engine, and listen right away before the ECU flags the misfire and cuts fuel. Otherwise you'll think it is a bad injector, and it isn't.
Letting a car sit for months on end is not a good thing. I'd tell your brother to try and take it for a spin at least once a month, weather permitting, and try and keep the fuel fresh. Unlike diesel, gasoline starts to go stale in a matter of weeks, and after a month even in a sealed system like a modern car, has already lost several octane points. Ethanol laden fuel is even worse.