Sad to hear this as from the account it sounds like a stroke or perhaps OD.

"Weir appeared OK at first. But during a second set that included “Eyes of The World,” “Uncle John’s Band, “Scarlet Begonias” and “Unbroken Chain,” Weir could barely remember the words to several songs and was increasingly unable to stand up.

He nearly fell into the amps behind him at one point before tense crew members brought out a red folding chair and placed it in front of Weir as the band played on. Weir kicked the chair and continued to play guitar standing up. He then swayed to his left and fell down, stunning the audience.

With the help of crew members, he was assisted up and sat in a red folding chair before nearly falling over several times.

Eventually, after Weir began to play something on the guitar by himself, the crew unplugged his guitar. Lesh and bandmate John Kadlecek walked off stage, leaving Weir alone before he was escorted off by the crew."

[link to zagsblog.com]