No charges were laid against guard Thomas Driscoll in the 2010 incident, and Vancouver police later determined he abused authority, and they ordered a written reprimand and retraining.
The B.C. Police Complaints Commissioner then appointed former provincial court judge Alan Filmer to review the case.
In a ruling issued Wednesday, Filmer said charges of assault causing bodily harm or assault with a weapon should have been laid against the man.
"The complainant, handcuffed behind his back, was completely vulnerable and offered only mild resistance," Filmer said of the assault.
"The assault, it is suggested, resulted either from frustration on the part of Driscoll or 'to teach the complainant a lesson.'"
The ruling said Driscoll used a four-inch handcuff key, sticking out from between the fingers of his right fist, to punch the complainant, Hayden Wood, injuring the victim's forearm.
"The blows caused small puncture wounds and slight bleeding."
Filmer said while Wood was intoxicated and abusive, Driscoll's actions were "at the very least, reckless."
While Driscoll no longer works at the jail, Filmer has ruled the man should receive a five-day suspension without pay in the matter.
He noted the lawyer for the police complaints commission also took issue with the suggestion that Driscoll accepted responsibility, because the man didn't report his conduct, didn't fill out a use-of-force report and didn't offer first aid to the prisoner.