Does a rescuer who is injured helping someone have a negligence claim in Ontario?

Generally, yes. Ontario courts have long recognized that a person who is injured while reasonably attempting to rescue someone else from a danger created by a defendant's negligence can bring their own claim against that same negligent party, on the reasoning that a rescue attempt is a foreseeable response to a dangerous situation someone else caused. In other words, the defendant who negligently created the danger is treated as having also created the foreseeable risk that someone would step in to help, and owes a duty to that rescuer as well as to the original victim.

This protection exists in part because the law does not want to discourage people from helping others in genuine emergencies out of fear they would have no recourse if they were hurt while doing so. The rescue attempt does need to be a reasonable response to the danger, rather than something reckless or wildly disproportionate to the actual risk involved, and the danger being responded to needs to be real rather than imagined. A rescuer's own conduct during the rescue can still be assessed for contributory negligence, which can reduce - but does not automatically eliminate - their damages if part of what they did was itself unreasonable.