Matthew Perry’s longtime assistant faces sentencing Wednesday for supplying and injecting the ketamine that caused the “Friends” star’s death. LOS ANGELES — The final sentencing in the 2 1/2-year investigation and prosecution following the drug death of Matthew Perry will be of the personal assistant who sat at the center of the entire affair, buying the ketamine that would cause the death of the “Friends” star and injecting him with the lethal dose. Kenneth Iwamasa, 60, is set to be sentenced Wednesday in the Los Angeles federal courtroom of Judge Sherilyn Peace Garnett, who has sentenced four of his co-defendants…