Complex sentence: Frey then went to Los Angeles to reconnect with his girlfriend, and he was introduced to J. D. Souther by her sister, Alexandra Sliwin, who was Souther's girlfriend at the time.
Split into simpler sentences: Frey then went to Los Angeles to reconnect with his girlfriend. He was introduced to J. D. Souther by her sister, Alexandra Sliwin, who was Souther's girlfriend at the time.