Even if the show wants to do something to address some of the huge multitude of realities about addiction, there is no way this can even begin to be done responsibly in a single episode of Glee. And there is no way to do this through Finn’s story, since Finn’s entire story is the radical opposite of Cory’s when it comes to the life and the factors that led Cory to do drugs.
Cory was the product of a broken home where his dad first abandoned the family when Cory was 18 months, and then was in and out of their lives throughout Cory’s boyhood, separating or divorcing his mother when Cory was 7 or 8, and disappearing altogether when Cory was 12 — which is the time when Cory started to abuse substances. From the ages of 13 to 16 Cory was in and out of 16 different reform schools, getting in more trouble all the time, breaking every rule he could, and moving to worse and worse substances. He dropped out of school altogether at age 16, and spent the next 3 years from ages 16 to 19 — 3 of the 4 years in age that we’ve known Finn Hudson — committing crimes to get money for drugs, all but living on the streets, and doing “anything and everything” when it came to substances he abused. It was a life of chaos and horror and pain. His family forced him into rehab; he went, came out, and immediately got into trouble again until he stole a bunch of money from a relative in a cry for help and finally managed to commit to being sober, and then over the next decade turned his life around, found a dream, and worked tirelessly to pursue it, finally winning huge success.
Finn lived in a home believing his dad had died a hero when Finn was a baby. He had a wonderful, loving, strong relationship with his mother. From the time he was a little boy, he wanted nothing more than to make his mother proud because he realized the sacrifices she made for him.
In the years we saw him — ages 15 - 19, the age when Cory was at his most out of control — Finn was the epitome of being responsible, of being solid, of being dependable, and of keeping it together. He was responsible for his football team and for the glee team. Responsible in immediately committing to step up to the plate when he thought he was going to be a teenage dad, from unfailingly supporting his girlfriend to wanting to be a part of the pregnancy to responding to her berating him to earn money to pay for medical expenses by finding a job; he worked hard to put aside his feelings for another girl because he felt compelled to stand by the girl he thought he’d gotten pregnant.
He worked hard to be a leader; he worked hard to be a friend. His story was one of progressively stepping up to the plate as a leader, even though he faltered sometimes, and of standing by and for his friends and teammates. He came to accept his mother’s new relationship and his new family. Until the painfully (and certainly deliberately) written one-off meta story in 4x19, Finn never took a break from being the most responsible character on Glee; he was always the one working on holding things together for the rest, and we never saw him let loose and have fun or be anything other than super-concerned about trying to do the right thing.
He fell completely in love with Rachel, supported her in everything, believed in her in every way, and in the end sacrificed all of his happiness and the one thing he had going for him in his sorry life — her — so that she wouldn’t be held back from her dreams in sacrificing herself for him. And in his final year, he was on the path to devoting himself to a lifetime of putting himself on the sidelines while he lived for the purpose of supporting others and helping their dreams to come true. He never really went after a dream — he never even stated, in his own words, that he’d found a dream (at least not since 3x16 when he said acting was his dream). He never achieved a win; he never proved to himself that he was New York good.
None of the terrible, painful, hard, complex realities that Cory lived with, and that led him into the clutches of addiction as a child, were Finn’s realities. There is literally no way to tell Cory’s story with addiction through Finn’s life. Cory was not Finn. Finn was not Cory. Trying to put Finn into Cory’s story would be like trying to force Finn to wear a costume made for Rachel — it would be something that in no way could be made to fit him.
The show could possibly do an OOC episode about Finn dying from a drug overdose — although a final tribute episode is not the time to go OOC with one of your main characters; that’s the time, of all times, to be absolutely and utterly faithful to the story and characterization that has been told and developed for four years. There’s no way it can do a story about the reality of a lifetime struggle with addiction because that is in no way, shape, or form Finn’s story, and there is no believable way to make that his story in retrospect. To try to do so — to try to say that Finn’s been a drug addict for years, or even months, and we just never realized it — would, in a single episode, stretch credulity to the point where it would irrevocably destroy the show. And it would be an irresponsible way to address the realities and complexities of struggling with the disease of addiction.
Reducing either Cory’s life, or Finn’s life, to a single issue would be both to disastrously undermine the entire story of Glee, and Finn’s role and position and purpose in Glee from the start, and to gravely disrespect the life of an actor who was so much more than his disease, not to mention disrespecting the complexity of his struggles with that disease.
It would not be a way to pay tribute to a character and an actor; it would be using a character to deliver a simplistic didactic lesson and so dismiss him from the scene.