I agree with that approach but be aware that some plasma cutters include a regulator that is controlled automatically and will not fire (they display an error) if the air pressure is low. The definition of "low" depends on the make but it is often above 70psi - I think mine is just under 90psi.Charge up the compressor tank, before plasma cutting, and time how long the compressor feeds 70 psi for, turned off.
Cutters with a built in compressor are usually limited in the thickness that can be cut when using the internal compressor. I don't know what the requirement is but that might be an option.Why doesn't he get one of the Lidl plasmas with the built in compressor? Should fit the bill and run off the genny. Clarke also do one with a built in compressor although it will cost a lot more.
If the work can't be moved closer to a power point, it might be better to get a bigger generator. Otherwise, make sure the plasma cutter is only set to the current needed to make a clean cut - that will minimise the power requirements. If you do that and you want precision, you might need some smaller tips as well.
Jack