You need to take it to another mechanic
They should be checking the actual pressure and desired pressure. Return on the pump, pressure relief and each injector. Replace any out of spec. If everything is within spec then it is not the fuel system and possibly something else like cam or crank sensors, electrical, etc
Does it smoke at all? Proper cranking RPM? A good mechanic should know all this. Get your money back and take it elsewhere. Let us know where you are located. Maybe we have recommendations for a good shop near you