The friend -- an enigmatic celebrity sidekick named Raffles van Exel -- tells us, he is "neither the architect nor the culprit of the sudden demise of Whitney Houston."

Raffles was with Whitney in the hours before she died at the Beverly Hilton hotel in February -- and there were widespread reports strongly insinuating he was the singer's secret cocaine supplier.

There were other reports suggesting Van Exel was also the one who snapped the infamous picture of Whitney's body in a coffin ... and sold it to a tabloid newspaper.

But for the first time ever, Van Exel is adamantly denying all the reports -- and has begun taking aggressive legal steps to save his reputation.

Van Exel has started firing off cease and desist letters to media outlets reporting any nefarious connection between him and Whitney's death -- demanding full retractions asap ... or else.