To get the generator running, your friend will need a 12V battery and an AC powered battery charger as the generator most likely doesn't have a battery charge circuit built in. (It's an option on at least some of the Onan RV generators, rarely fitted.) Probably it's unnecessary to worry about the other wires for the inside remote control and hour counter and pilot light for basic operation; the start and stop switch on the generator I think should work just fine with them left unconnected. Of course, having a remote panel would probably be more convenient.
I'd think an old but not completely dead car battery, or a not old riding lawn mower battery, would suffice for starting the generator.
