There's a lot of different issues going around here. Friends; I think my answers are different with friends. I suppose it matters what friend, but I don't feel I have the same obligations to strangers that I do to my kid(s). I might take a different approach with them depending on the crime. The trading cards? Not my business, not my conscience, not my obligation to teach. If one of my friends raped or killed someone? I don't know, and I may be able to assuage my conscience in different ways (anonymous tip, maybe?)
As for grandpa, not to get all existential here, but having lost both parents in the last 18 months, I have a much different view of that time of the life. Not that I'm any less scared of death or any less enamored with life, but at the end, there's a real "come to Jesus" (pardon the pun) concept about what matters. I don't regret, per se, but one of the things I wrestled with near the end of my dad's life was "letting him" go back to his condo. When he fell (some of you know the story; he shattered his leg in November, went to rehab then into assisted living and died the following June) it was difficult for him to accept that his circumstances changed. My brother and I resisted letting him go back to his condo, because mentally he wasn't ready. In the end, the day he fell was the last time he ever set foot in his condo. I felt bad about that for a long time, but in hindsight, he still got to spend every minute of the remainder of his life with his beloved wife, and we - my brother and I - got to make peace with them and say all the things we needed to say. The material things he was concerned with at the end don't matter at all now and I believe that he went out on his own terms (some of you know that as well: his cancer that he beat twice came back with a vengeance, and he died literally in the transport on the way to hospice. I will take to my grave that he knew he'd never see his wife again, or drive his car, or anything else and simply said "enough; this isn't the life I want to lead".
If my dad killed someone (rape was probably off the table) at that point, I would probably urge him to do the right thing vis-a-vis the victims, but as much as I am beholden to our rule of law, or the US justice system, what is that going to prove? No lessons were to be learned at that point; I've talked about the victim a lot, but "vengeance" is not my priority even in the slightest, it is human decency. Past that, family first.