And if there were mentoring relationships going on, both sets of parents would be aware of it. No young person of 12, 13, 14 etc. needs to be turning to another teenager for help, sorry. The help comes from adults, not other teens, and especially not someone outside of one’s family, who is unknown to the parents. If a mentor is needed, it is the parents’ job to find that person or to screen them. Kids cannot help other kids understand the world. A 17 year old with the right mentality would say (or write) to a 12 year old, “You know, you really should talk to your parents about these things. I’m sure they want to help you and they care about you and your future.”
Technology has created this opportunity, the clandestine pseudo-relationship between strangers. What troubles me is that so many teenagers (or even children) seem to think that someone they chat with on Facebook is a real friend. These 2 people have never even met, yet they are swearing they LOVE each other? I can recall at least 2 recent threads by people who contacted someone through a dating website, seemed to hit it off, then met the person and immediately knew there was nothing there that they wanted to pursue. It is a FALSE impression of a relationship.
My intuition of what has transpired in this case is based on different factors than your evaluation has been. I would love to hear what was in that letter, and what the father has to say. I am glad that he blocked all of her communication and that his son has thus far obeyed and closed off avenues, even though this young woman tried to sneak around that by going through a friend.
OP, I hope you come to grips with what has happened and that you stop contacting this young man. Let him get on with his life and you should get on with yours.