...moral philosophy?

I had a one-day trip recently where I knew I'd have to spend much time in airports and sitting in a café by myself, waiting. Naturally, I bought a novel to pass the time. I'm glad to say I almost finished it that same day, because I enjoyed the story telling and the on-and-off-again philosophical musings of the leading heroine. The novel is The Sunday Philosophy Club by Alexander McCall Smith, if you intend to read it may I suggest you stop reading this entry now as I will tell you the end.
The story is of a woman, editor of a philosophical magazine, witnessing a young man falling to his death in the concert hall. We get to know her, hear about her love-life (virtually inexistent, but serves as reference point for some philosophical musings), learn about her need to lead a moral, good life and the resulting curiosity and sense of obligation to find out whether the young man really fell to his death or was, indeed, pushed. As I said, I liked the story-telling and especially the Edinburghian scenery, but I was rather unsatisfied with the ending. Take a hint, this is your last chance to go and read the book without knowing what will happen. ;-)
Of course she finds out that the guy was pushed. She has her suspicions throughout the whole novel, finding new theories and meeting new possible suspects in every other chapter. Still, I wouldn't call the novel action-packed or creating great tensions or suspense, as other criminal novels might. This ending comes more or less expectedly, she goes and talks to the guy who she knows was at the scene and had lied to her before. She finds out that said guy indeed did push his friend, but that push was neither on purpose nor intended to end in the way it did, in short: it was an accident. Tragic, but "true". And what does our heroine do? This is the bit that left me with a furrowed brow: She decides a) that her moral obligation is fulfilled, and b) that since the pusher convinced her of the accidental nature of the tragedy, it is unnecessary to inform the law enforcement authorities of the new facts concerning the case.
Now, is that moral? To blatently ignore rules set up by a democratic society based on laws? Is it really for the heroine to decide whether the pusher has received all the punishment he needs? Wouldn't that be for a court of law or a jury to decide? Plus, she hardly knows the guy! I mean, what if the pusher wakes up one morning and decides he can't live with the fear of being uncovered, and ups and kills our heroine? Just because she decided at this one point not to report him, doesn't mean she won't change her mind, right?
In other words, is our moral obligation to the individual or to society as a whole?
I learned from that novel that it is perfectly acceptable to ask questions one doesn't know the answer to, so this is exactly what this post is. I wouldn't know what I would do in a similar situation. Say a good friend of mine confesses to murder, would I report him or her? Or would I believe in the years of friendship and trust my instinct if it tells me that what happened was an accident? You know what? My gut instinct now tells me I'd report them. Not because I'd love them less or not trust them anymore, but because I wouldn't want to know something that I can't share with anyone. I mean, I'm alright with keeping the little secrets, but big stuff like this would be weighing too heavily on my troubled mind. What if somebody else found out, would they construct my knowledge as accessory to murder? So, in real terms, would I just be wanting to save my own skin by blabbing? Very likely, but not morally reproachable, I would hope. Well, I'd also hope that my friends know me well enough never to put me in a position where I'll have to choose between them or my own conscience. Let this be a warning to yous, and a promise, I'd do everything in my power to get you the best possible legal representation.