I hate it when things like this happen

I'll withhold my opinions of the people involved and cut straight to the advice. First, judging from your fluency in English, you are probably in an English-speaking country. If not, the following advice might not apply. Secondly, save the letter in encrypted form on your computer, never print it, let alone send it to anyone. If you show any interest in her, or any anger or even disappointment at your friendship with her being destroyed, it will only make the bastards even more sure they did the right thing.
Sorry, but I think that there's not one damned thing you can do now. Talking to the mother or writing a letter or trying to contact the girl directly, all dangerous and pointless. Maybe in 8 or 9 years you can locate her and give her the letter and try to convince her that her parents (or just her mother) destroyed something wonderful that would have done her a lot of good.
But even if you accomplish that, all you'll have is a 20-year-old woman friend, and is that really worth the effort? Not in my opinion, but it's up to you. Run, don't walk, away, and if you look back, they'll know you deserve all the abuse they can heap on you.

Oh, by the way, if I sound bitterly angry, it's because your post has reminded me of all the times beyond counting that I was bitterly angry for the exact same reasons. I have experience in trying to salvage something from things like this, and that's why I can say it isn't likely to work.

Ultimately, though, it's your call.