I saw him on the final tour. His mind was wandering and he sometimes forgot the words but he could still sing and he could still play the guitar. In fact, sometimes, to find his way back to the song he’d do a guitar solo. Apparently that part of the brain hadn’t yet been effected. Funny how the mind works.
BTW, anyone who doesn’t already know this should look up Campbell’s work with the Wrecking Crew, back in the 60s. He was one of the best rock guitarists going, bar none.
Yes, Campbell played guitar on all of the Beach Boys hits in the 1960s, plus many, many, other classic pop hits for Elvis, etc.