Generally speaking, a car should be serviced every 12 months or 12,000 miles (whichever comes first). Failure to adhere to the service schedule will ultimately invalidate the manufacturer's warranty, so it's very important to keep to Citroen's recommended schedule. The dealer is correct about the service plan (it starts from the date of registration) but I would argue that this important piece of information should have been explained at the point of sale. Your friend should make a complaint to the dealership. I would expect them to refund the cost of the first service.