Hi, there.........I'm so sorry for your grief and confusion, but reaching out and taking ownership of your personal choices is HUGE, on your part, because it is an effort to "do something" instead of pretending or ignoring the core issues.

With regard to the childhood friend and the disappointment that he didn't turn out as expected, that just is what it is.  If he is "alone" in his fight with cancer, it is not your "fault" or "responsibility," by any stretch of the imagination.  Something about this person caused you to feel alarmed enough to get the heck away from him, and the fact that he has a dreadful disease has nothing to do with it.  Just for your own edification, remove his struggle with cancer from the equation:  if he were NOT "dying" from this disease, would he be a different person?  Probably not.  Drug addiction tends to ravage a person, on every level, but the addiction is usually a symptom of some underlying disorder.  So..........this guy would have been toxic to you, cancer or not.

I'm sensing a tremendous burden of guilt, and it's very easy to suggest to someone to let go of the guilt, but quite another thing to accomplish it.  We are all human, we all make mistakes in judgment, we all do things that we are not proud of, and I'm getting this sense that you're feeling guilty for his struggle with cancer - that you should, somehow, "be there" for this person, EVEN THOUGH your fight-or-flight instinct is telling you that he is a danger to you.  Perhaps, if you accept that this guy is what he is and that you are not responsible for WHAT he is, you can not only let go of your guilt, but also the fantasy of what you expected. 

I wrote this parable in another response, but it's appropriate here, I think.  A man is at a shoreline getting ready to cross a body of water in his boat.  He sees a rattlesnake moving back and forth at the water's edge and the snake says to him, "Please, I need to get to the other side.  Will you take me with you in your boat?"  The guy responds, "But, you're poisonous."  The snake reassures him, "I promise I won't bite you if you take me across.  I promise."  So, the man agrees and, as he lands on the opposite shore, the rattlesnake bites him.  "But, you promised you wouldn't bite me!" he cried.  The snake said, "What did you expect?  I'm a snake, and that's what I do." 

The point is that, for whatever reason, something was lacking in your life enough for you to build a story around someone else.  Certainly, feel compassion for this person who is battling cancer, but attending to your Self (Self = "soul") and recovery/healing your own Self is primary, right now.  That can be done through conventional counseling therapy, or via group support, or in many other ways.  But, it might be helpful to determine why you feel compelled to base your own comfort and happiness on someone else.  When WE are confident, self-assured, and self-validating, we don't allow toxic people into our lives, regardless of their personal situation.  We pick and choose whom we allow, and we don't make ourselves "feel badly" about making those choices, or maintaining our personal boundaries.

On a strict aside, here, I was in contact with a former love from 35 years ago.  Out of the blue, we communicated, and I had always felt SAD that I had left this guy when I was in college.  I am an artist, and he didn't approve of what I was and made no bones about it.  And, he was also very possessive and controlling, so I walked away from him and always believed that he could have "changed" if he'd loved me enough.  Well, in this single conversation with him, 35 years later, he made it very clear that he was STILL the same person that he had been, and was still just as narcissistic, if not worse.  He said, "There's nothing wrong with two old friends chatting, is there?"  I thought, "Old friends?  We were lovers, and we had planned on marrying!  We were intimate for 2 years, and he's talking about 'old friends?'"  So, the young adult fantasy that I had maintained for 35 years about this man evaporated within the span of 30 minutes.  I couldn't change a snake into a kitten.........

Best wishes to you and most sincere bright blessings................