As soon as the girl steps on the shell, the first thing that happens is that the sensory neuron, which in this case is located on the skin, receives the information and transfer it to the inter-neuron or the association neuron which is located between the motor and the sensory neuron. From there, the motor neuron relays the information to the effector e.g a muscle that responds to the stimulus by removing the foot immediately.
The next time he goes surfing, he will get tired a lot faster and will run out of breath a lot faster. He will also feel a lot more tired quicker than he used to.