Interesting question ... and there is indeed a fine line.
but NO, it certainly was not your fault nor your responsibility. It was his. Although his death is labelled as 'accidental overdose', had he looked into the effects and side effects of mixing such prescription drugs and the amounts taken, he may have 'chosen' not to take them, or possibly looked for less toxic alternatives. His health and wellness is/was his responsibility.
We can only be responsible for our own actions.
Had you taken the time and effort to 'warn' him (and/or others) you and your information most likely would have been ignored.
The rest of this answer was sent to the enquirer, as way too long to post here (according to Hubpages)