The book is Hot Burritos - The True Story of the Flying Burrito Brothers by John Einarson with Chris Hillman.

Page 208, the Burritos were at the Altamont concert and when their set was done, Sneaky Pete, Chris and Jim Dickson left as the security force, the Hell's Angels, was beating up people in the crowd, and had actually come up on the stage when the Jefferson Airplane played and beat up Marty Balin. So the mood was ugly, it was now dark and the Rolling Stones delayed their performance by 2 hours. Gram Parsons(who worshipped the Stones blindly), Michael Clarke and Bernie Leadon stayed behind. Gram kept telling them that they would get to fly out of there in the helicopter with the Stones. When the Stones finally came out the lights came up but they only played a few songs. Bernie recalled being backstage facing the audience and fights breaking out and people would push away from it and then he would see pool cues coming down and somewhere in all this, that guy got killed, on camera and there was such a press of people that he got passed up and laid on the edge of the stage like a sacrifice. It was such an ugly vibe, the Angels were turning meaner, you could see it in their faces. Jagger stopped singing and tried to calm the Angels down but no deal. Gram left the stage with the Stones and ran to the helicopter and got on with them so Michael Clarke and Bernie Leadon were left behind. They started walking and met a couple of girls Bernie knew from L.A. and got in their car and wound up getting back to Sausalito.

That's a sort of summary of the article with a few quotes, not my own. But Gram by then was pretty far gone into drugs and alcohol and hero worship of the Stones to the exclusion of everyone else. I've got the movie Fallen Angel about him plus several books and Bernie is in the movie being interviewed along with Chris Hillman. Totally fascinating.
Wow, thank goodness Bernie was good with the ladies, lol. Seriously, how scary, though.