If your question were simply “Is a bystander who knowingly allows harm to occur guilty of something?” the answer would be easy: Yes. But that’s not what you asked. You asked if that person is as guilty as the person who commits the act itself. In the Penn State situation, I think there’s a social pressure to argue that almost every adult involved is equally responsible. But that’s not really accurate; you can certainly view every adult in that scenario as guilty, but they’re all guilty of different things. Unless you view “guilt” as a blanket, uniform classification that applies to all involved parties in every situation, you have to assess every act individually. And that means the (very bad) act of not stopping something harmful is not commensurate with the (very, very bad) act of actively committing harm.
Let’s use your original hypothetical, but with a reversed outcome: Say an ostensibly sane friend tells you he’s going to murder his rival, but you stop him from doing so (you take away his weapon and convince him that it’s a bad idea). Clearly, you’ve done something good. You stopped something bad from happening, and you helped maintain the social order. Your friend, however, can’t argue that he did something good by not killing his enemy. He doesn’t get credit simply for not shooting someone. So while you both share a degree of accountability for this positive nonevent, your respective roles have different values. Your pre-emptive action matters more than his inaction.
A similar principle is at play when the inverse happens. Let’s say that same friend tells you his plan, and you choose to do nothing. You just sit there silently, staring ahead blankly. His murder proceeds as planned. Now, you’ve absolutely done something unethical here; you behaved cowardly and apathetically, and you might be considered an accessory to the crime. You share a measure of responsibility — but a fraction as much as the murderer. Your failure was tangential. If you were totally eliminated from the equation, the violent event would still happen as it did; if your friend were removed from the equation, there is no event at all. Actions have to matter more than mere knowledge.
Your allusion to what occurred at Penn State fits into this model of thinking, but with a few extra levels of complexity. Jerry Sandusky committed sexual acts against children, so his culpability is not unclear. But it can’t be easily argued that the acts would still have happened even if Joe Paterno had never existed. It’s possible that Sandusky was able to commit his crimes only because of the culture Paterno helped create, so his role in the debacle (and the culpability of the institution) is less straightforward. Anytime this type of question is examined outside a vacuum, it becomes highly situational and increasingly intricate (sometimes the person committing the wrong understands the true consequence less accurately then the informed bystander, which implicates the latter individual to a much deeper degree). But right now, we’re still in the vacuum: You’re asking if not stopping a horrible event is the same as actively causing a horrible event. And it’s not.