Thank you

Regardless of what's actually wrong with it a faulty mass air flow sensor will not prevent an engine from starting or running.

The car with the problem in the same make and model as my car. Used an ELM327 to see any error codes.
Maf wasn't defective. when car is stoped, maf reaches 144g/s, and when it's running it drops to 6g/s at idle.
Found what could be the problem. Used elm to see Fuel Rail Pressure and my car has about 200kpa with engine stoped, and imediatly goes to about 30000kpa when i try to start the engine. The car with the starting problem has no increese in FRP. It stays at 800-1200kpa when I try to start the car, the same goes to when engine isn't cranking.
So at this point I'm looking for a faulty FRP sensor, faulty FP regulator or faulty fuel pump.

Since the car isn't mine and cant get it to a safe location where I can further determine the cause of the problem, I adviced my friend to go to a mechanic.

DO NOT be tempted to slacken the fuel injector pipes for any purpose. This will not get you anywhere and can be potentially dangerous. In addition you probably won't reseal them meaning new components will be required.

Check the fuel system between the fuel tank and the high pressure pump for any signs of air ingress. Check the fuel filter and housing. If you have an in-tank fuel pump make sure it's working. If you have a priming bulb under the bonnet check to make sure it isn't pulling air.

If all that's good then as said earlier, a worn pump, excessive injector leak off, stuck open relief valve or even faulty fuel pressure sensor could be the cause. The problem with common rails is it's almost impossible to tell if the rail pressure is good. Pressure comes from opposition to flow, and so it's the pump pushing fuel against the tiny injector nozzles that create it. You won't have hardly any pressure with the high pressure pipes slackened.