OK, concerning the hypothetical scenarios again, but with a stranger, not a friend...
I'd certainly do something about the drinking and driving. Maybe not once, but if he made a habit of it, I'd report him. Since nobody could do anything at that point because it's heresay, I might try to do something useful, like persuade his friends, wife, mother or anyone he might listen to, to stop him. If someone was killed I'd very definitely report him (or her), and be willing to stand up in court if necessary.
The hard core porn? I tend to think what people do in private is their own business, however distasteful. If it concerns kids, it's even more distasteful. But I don't know for certain that it leads to harming anyone. It may be against the law, but to me, that's not the point. I might mention it, so that he knew that I knew, but no more, unless it seemed that he might do more than just look at pictures...
Low level flying. I think our low level flying rules go over the top. I've been in the US, flying legally at 200-300 ft...in helicopters, don't know if f/w rules are the same there or not. It didn't do anyone any harm, it wasn't dangerous, and often that's the case here. If it was a one-off, I'd probably ignore it. If it happened regularly, I'd mention it to someone the guy/gal might be inclined to listen to...flying instructor, airfield manager, or whoever. If there was an accident? This gets complicated. To what extent are you responsible if an accident is caused by something you do, but which you didn't directly cause? If I walk naked down the hard shoulder and an accident occurs, was it my fault? If I have a super-expensive flashy car that everyone gawks at and the staring and lack of attention to driving causes an accident, is that my fault?
I think this last example demonstrates clearly the reason for the differing opinions on this thread. Low flying is not dangerous, in and of itself, all the time, by every pilot. It just happens to be against the law in the UK. And I personally would ALWAYS do something about things which are dangerous or could harm others, even if they're legal (mild neglect of animals falls into this category, and I've made enemies as a result in the past, and I don't care). I won't step in just because something is against the law; sometimes, IMHO, the law is an ass. Low flying is a borderline case; hence the rather (I hope uncharacteristically) woolly thinking in this post.
This is my thinking so far. But none of this is clear-cut black and white, as you know. If I manage to come to a clearer view, I'll let you know.