Here is a great opinion out of California that does a nice job of handling the "causation" issue in a case against a security guard company that is alleged to have failed to provide proper protection to a c-store employee.
The case is Mukthar v. Latin American Security Company, B183968 (Cal App. 2nd Div. 5/8/06).
An excerpt: "We disagree with the trial court that it is conjectural whether a "security guard could have prevented the attack on the Plaintiff." The issue is whether it is a question of fact whether the woman would have struck Mukthar in the face, if an armed, uniformed security guard, equipped with a baton and handcuffs, would have stood next to Mukthar. (There is no dispute about the fact that the guard's station was at the door, where Mukthar was standing when he was struck.) We think the inferences are not evenly balanced on this issue. It is more likely than not that the woman would not have hit Mukthar in the face in the close proximity of an armed guard who had the ready means at hand to respond physically to violence. Be that as it may, it is not for us to decide this question of fact, which is consigned to the trier of fact."
Thanks to Shaun Martin at California Appellate Report.