'Diff'rent Strokes' star Dana Plato's suicide left her son in a world of "pure hell," which ended in his tragic May 6 gunshot suicide, his grandmother tells PEOPLE.

Joni Richardson said her 25-year-old grandson (pictured above in 1999) struggled with drugs and alcohol for 10 years, desperately attempting to "drown out the pain." His mother took her life with prescription pills when he was just 14.

While there has been speculation Lambert was on drugs at the time of his death, the Tulsa, Oklahoma Office of the Chief Medical Examiner told PopEater toxicology tests could take two to three months.