I don't see why not, I have done so. Just make sure to use wiring rated at the correct current level for the proposed maximum current draw, and have the fuse rated below this level. Fuses are there to protect the wiring, so there's no point having a 15A fuse and wiring rated at 10A.
Since the socket will be live at all times, your friend will also need to ensure there is some form of safety cap over the socket to prevent accidental short-circuit and possible fire if anything inappropriate gets poked into the socket, and provide some means of ensuring they don't leave anything connected to it when the engine is not running, to avoid flattening the battery.
