I was thinking the same, a tank. All compressors I've seen have the unloader built is a part of the pressure switch, So as soon as the compressor is powered up, the decompression valve is closed. Compressor wouldn't even budge if it had a head pressure of 90 psi.
I have an old 2 stage air compressor, guessing 60 years old, that has a set a weights on pulley and as the compressor comes up to speed, the weights move, and a lever closes the decompression valve on the pump. This compressor has a unique motor too, no capacitors. It's a repulsion start, induction motor. The brushes lift off the commutator when motor comes up to speed.
I'm still betting the compressor is an oil-less and my limited experience says they have a large LRA. Also betting that only half the generator is feeding the 110 outlet. So he is really only getting half the power to the compressor, not enough. He should look at the round plug for a 110 volts and a larger amp rating.