Complex sentence: Her dalliance with studio chief Mack Sennett famously ended his engagement to actress Mabel Normand--who had actually been Busch's mentor and friend--when Normand walked in on the pair.
Split into simpler sentences: Her dalliance with studio chief Mack Sennett famously ended his engagement to actress Mabel Normand. Normand had actually been Busch's mentor and friend, when Normand walked in on the pair.