His fuel accumulator should be maintaining pressure in the system indefinitely. He wouldn't really need to put the key in the on position and wait for any interval of time at all.
If his system is not holding pressure overnight, then the problem would surely by the accumulator.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".