Black & white? What does that have to do with the OP’s question?
He wants to know if he should allow his son to befriend his OTD friend. And the answer is, no, stay away like the plague. And yes, YELL IT OUT TO EMPHASIZE THE POINT. There is NO BENEFIT, I repeat, NO BENEFIT to the son in upkeeping a relationship with a close peer that is into drugs and the L-rd knows what else.
I agree, it’s sad that this happened, but for the son’s sake DISCOURAGE him from maintaining this relationship.