Monteith took the deadliest possible combination—alcohol and heroin, whose actions to slow breathing are not additive but multiple—at the deadliest possible time. He was likely not informed about the risk because abstinence-focused rehabs typically don’t provide harm reduction advice. He certainly was not provided with maintenance medication like methadone or buprenorphine that can dramatically reduce that risk; he may not even have know that maintenance was an option—just as Cobain was told he could not take any more opioids, even for his chronic pain. Nor, apparently, were Monteith or his loved ones given naloxone, which can reverse opioid overdose, or instructed on how to use it.
The vocab acrobatics in this are pretty hilarious. “The lowest mortality risks were among those in the overweight category (B.M.I.s of 25 to 30), while moderate obesity (30 to 35) offered no more risk than being in the normal-weight category.” So “normal” sounds really meaningful then ;)