"We are in complete shock," a family friend tells PEOPLE in this week's issue. "Those who were with him in his last days saw no sign of [distress]. It has gutted us all."
In fact, the Oscar winner had been eagerly looking ahead, excited about Aug. 23's Hotbed Benefit fundraiser for 10,000 Degrees, an education charity in the San Francisco Bay area where he lived.
Williams's artist wife, Susan Schneider, was donating several of her paintings for the event, and Williams himself had contributed one of his favorite bicycles.
"He was so excited to be a part of it," says the source. "Susan was going to be a featured artist, and he was so supportive of her."
The Saturday benefit, which is to be hosted by Kathy Griffin and sponsored by The Drever Family Foundation, will now pay tribute to the late comic.
Reporting by ALICIA DENNIS
For much more on how Robin Williams's friends and family are coping, pick up this week's issue of PEOPLE Magazine, on newsstands Friday
The Williams family is asking well-wishers to send contributions to charities close to the actor's heart in lieu of flowers. Suggested organizations include St. Jude Children's Research Hospital, Challenged Athletes, USO, the Muhammad Ali Parkinson Center, the Christopher and Dana Reeve Foundation and Glide Memorial Church in San Francisco.