Complex sentence: When Sullivan drops into the Dior fashion show one day to look for Joan, the staff have never heard of her, however he sees her observing the show with her father's friend the Senator, whom he mistakenly assumes must be her sugar daddy.
Split into simpler sentences: When Sullivan drops into the Dior fashion show one day to look for Joan, he discovers that the staff have never heard of her. However he sees Joan observing the show with her father's friend, the Senator, whom he mistakenly assumes must be her sugar daddy.