What is the test for establishing a cause of action against a private security company?

Causation must be established on a balance of probabilities before damages are assessed. In this regard, McLachlin C.J.C. stated in Blackwater v. Plint, 2005 SCC 58 at para. 78:

…Even though there may be several tortious and non-tortious causes of injury, so long as the defendant’s act is a cause of the plaintiff’s damage, the defendant is fully liable for that damage. The rules of damages then consider what the original position of the plaintiff would have been. The governing principle is that the defendant need not put the plaintiff in a better position than his original position and should not compensate the plaintiff for any damages he would have suffered anyway: [Athey v. Leonati]. …