Even if he kept the system on he would have problems with freezing. -15C is quite cold, so unless he's running a 15C+ water delta the water temperature will go below 0, at which point it will turn into ice. While the motion of the water will prevent the entire loop from freezing, ice will form in places and it could cause permanent damage to your WC gear. This is much like how a flowing river usually freezes over except for a small part in the winter.
Plus, if the RAD is mounted to the outside of the case it would become like a brick of ice due to the small tubes in the RAD trying to transfer heat.
He would need to run with the arctic glycol coolant mixture in his loop. The arctic glycols can keep the loop running down to -50. However, the condensation issue would be the next thing to account for.
I run my loop with a 1-gallon glass pickle jar. In the winter, the jar sits up next to the outside window glass. With the cold glass and the draft of the window, I do not need to place the glass jar oustide for extra cooling power.