If one does not do anything about it, (s)he is in collusion and it is exactly as if (s)he took the bribe directly. Blow the whistle. That said, I think we need to open a discussion on how unbelievably difficult that really is. We have relationships with those with whom we work; our livelihoods are attached to this; there is a ton of uncertainty of how it would all play out.
Few of us can truly predict with any degree of accuracy our future behavior when faced with stressful events. We like to think we can but it has been shown too often we stink at it. It is far too easy to turn a blind eye, justify/minimize it away, wait for the next guy to blow the whistle, and even distort the events in your head to remove the dissonance one might feel after witnessing it.
We all know what should be done, but the real question is how. How do you drum up the strength, how do you resolve the fear of the unknown results, how do you really take that first step?