Ok, calling someone stupid is unhelpful, and does not answer the question.
Logbook Loans do secure the vehicle on the car. Them holding the logbook is purely a conceptual thing and does not in itself give them title to the car. This is done via a "bill of sale" which your friend will have signed along with the loan agreement. The car will be registered with HPI as Logbook Loans holding a financial interest in it and as such it is not lawful for your friend to sell the car to someone else as this will not cancel Logbook Loans interest in the vehicle.
The simple answer is similar to those already posted - your friend cannot sell the car, as technically, until the security is satisfied (the loan completed), it's not his to sell.