That leaves the instructor with a gap he can't fill and if its the only trip he turned up for a wasted day. If the penalty is enough to cover the instructors pay then good.
If we assume the cost of a lesson to be broken down to aircraft hourly rate +instructor hourly rate.
I think it would be reasonable to charge a penalty the portion of salary for the instructor, however it would be unreasonable to charge a penalty for the aircraft, because in terms of instruction, an aircraft only costs money when being used (fuel, engine wear ...) ....
Just my 2p worth.