Complex sentence: When Carl permanently returned to the Beach Boys in May 1982, he asked Foskett to stay, which Foskett did and toured with the group for a decade until July 4,1990, after which Jeff left for his own solo career.
Split into simpler sentences: When Carl permanently returned to the Beach Boys in May 1982, he asked Foskett to stay, which Foskett did. Foskett toured with the group until July 4,1990, after which he left for a solo career.