Jerry used to play on recordings with many other artists, and this is one of them. I own Joan´s LP One Day at a Time from 1969 on Vanguard, and he plays fingerpicking guitar on a couple of numbers. Here´s a sample of her own fine picking on Sweet Sir Galahad from Woodstock 1969. It sounds very much like her recording.
http://www.youtube.com/watch?v=S4y2equAIdI&NR=1