Law enforcement sources say Matthew Lee, who was at Dodger Stadium with Stow the day of the attack, died Sunday after eating a salad that contained nuts.
Lee, who himself was assaulted, was considered an important witness and it is unclear how his death will impact the case.
Prosecutors say 29-year-old Louie Sanchez led a "belligerent rampage" against a number of Giants fans during and after the March 31 season opener in Los Angeles.
Sanchez and 30-year-old Marvin Norwood, were arrested July 21 and charged the next day with attacking Stow.
Stow, a 42-year-old Santa Cruz paramedic and father of two, remains hospitalized in San Francisco.
LAPD's Robbery-Homicide Division detectives are looking for additional San Francisco Giants fans from the bay area who were assaulted at Dodgers Stadium the night of the Bryan Stow beating. Calls should be directed to 1-877-LAPD-24-7. Anyone wishing to remain anonymous should call Crime Stoppers at 1-800-222-TIPS (800-222-8477). Tipsters may also contact Crime Stoppers by texting to phone number 274637 (C-R-I-M-E-S on most keypads) with a cell phone. All text messages should begin with the letters "LAPD." Tipsters may also go to LAPDOnline.org, click on "webtips" and follow the prompts.