Complex sentence: Before the party, as Fowler was checking to make sure everything would be ready for the 300 invited guests, he found Lillian sweeping the decks of debris and joined in to help her.
Split into simpler sentences: Before the party, as Fowler was checking to make sure everything would be ready for the 300 invited guests. He found Lillian sweeping the decks of debris, and joined in to help her.