When someone cheats who is at fault, would you say?

A likely scenario is that both share some degree of fault, as people below mentioned things like neglect. Though it is possible for the non cheater to be completely without fault. That is not a defense of the cheater though.

It is absolute that the cheater holds fault, and even when fault is shared the cheater holds the larger portion of the blame. Even in scenarios with neglect, where both parties hold fault, the cheater still holds more blame.