Nolan Wells' best friend stresses that he, unquestionably, didn't kill the teen ... or have anything to do with his death.

Brandon Tatum asked Nolan's buddy Warren Hudson point-blank if he was responsible for the teen's tragic, mysterious death ... and he said no. Full stop.

Warren also denied deleting anything off Nolan's phone, after his parents raised concerns that messages had been wiped.

Warren was MIA at the funeral Monday ... explaining he didn't want to risk his safety after getting "thousands" of death threats.

As you know, Nolan went missing after taking a boat out to Horn Island with friends on the Fourth of July. His body was found days later in the water.

Nolan's family is conducting an independent investigation, hoping to get answers to the mysterious circumstances surrounding their son's tragic death.