My cousin has just put £7 worth of unleaded petrol in a diesel car, he reckons there was about a five quids worth left of diesel in the tank before and then drove 500m to my house. Can he leave the car on my drive for a couple of days with the petrol cap off and hope it evaporates, nip back up to the garage and fill up with diesel and everything will be hunky dory? Or does he need to call out a mobile mechanic to have it drained?