They are known for drinking oil (as much as 1 litre per 1500 miles) something to do with the way the engines are run in apparently. The ones driven hard at the start of their life don't drink so much oil. If your mate is using fully synth oil, suggest they switch to semi-synthetic, it's supposed to help a little. Every 2 - 5 days does seem excessive though, most obvious cause would be a leak, can't always rely on looking for drips on the ground.. have a good look around the engine, usually around the rocker gasket.