Complex sentence: Shurley also noted that Genie's was the most severe case of isolation he had ever studied or heard about--which he maintained more than 20 years later--and offered several suggestions about how to work with her.
Split into simpler sentences: Shurley also noted that Genie's was the most severe case of isolation he had ever studied or heard about, which he maintained more than 20 years later. Throughout the time scientists studied Genie, he offered several suggestions about how to work with her.