So not only is he a pedophile, but he lied to you for his own benefit (downplaying how much porn he had so you’d write a letter on his behalf). Not really the kind of person I’d want to be friends with. If I were you, I’d be pretty upset he lied about how serious it was to get you to help him. You put your reputation on the line for him by writing that letter.
It’s sad that his mother is hurting, and if it were a less serious crime I’d be all in favor of staying in touch with him. However, one reason I would refuse to stay friends with someone like this is because I would never want to risk that they might molest a child in my family. I’d be as nice as possible to the mom out of sympathy for her, but I don’t think you’re obligated to hang out with a pedophile because you feel bad for their mom. He’s the one who chose to do this.
Yeah, child porn is bad, but the latter is almost more significant to me in this case. I’m willing to forgive almost anything, but if you lie to me to make me stick my own neck out, well, that’s a big, big problem.
I’ve known somebody brought up on similar (but not quite as overwhelming) charges. My belief is that he wasn’t a pedophile nearly as much as he was obsessed with taboo information. He wasn’t playing vigilante and didn’t claim to be, but I think he was fascination by child porn, not children. Still…not good.
Yes it’s sad that his mother is upset. However, your involvement is not her decision to make, it’s yours.
In exchange for your support so far, he lied to you about the charges to get you to write a letter on his behalf, and you still don’t know the full extent of it, correct? For that amount of bail and that sentence, it sounds like it was pretty serious. I would not put it past him to risk trouble for you in the future if it benefits him again.
I would back off out of this whole situation and cut ties with these people. I require a certain level of quality of the people in my world. This was no misfortune or tragic accident. It’s what he chose to do. Good luck.
I don’t envy you 
I also don’t think you should feel bad if you want to end the friendship.
I have no expertise in this subject, but a $600K bond for just posessing child porn? That seems more in line with what an extremely dangerous person would get. Six years in prison also seems a bit excessive (to me.) Are you sure he wasn’t involved in the creation of the child porn?
As others have said, if he was truly a victim of someone hacking his computer, he wouldn’t have lied and said, “oh, it was only a few pics,” rather than about a ton of them, AND a video slideshow. shudder
Yes, his mother is concerned about him, and asks that you be there for her son. But have either of them appologized for lying? For manipulating you for their own gain? I understand her feelings, but isn’t she concerned about what her son did? Has she said anything at all about that?
And finally, at least for me, it would always be in the back of my mind, “This guy has downloaded and looked at kiddie porn”. That would decide the whole thing for me.
I’d drop these people like a bad habit. He may or may not make it, but you don’t have to help him at the expense of yourself. And while you’ve been there for him in rough spots, he and his family sure don’t seem to care about YOU getting yourself involved in this. (The letter, for example)
That’s an excellent point, Guin.