Complex sentence: She remains at SCDP, however, and briefly becomes Lou Avery's secretary, after he complains about Dawn's loyalty to Don, and requests someone else, and she eventually becomes Don's secretary, when he returns to the agency.
Split into simpler sentences: She remains at SCDP, however, and briefly becomes Lou Avery's secretary after he complains about Dawn's loyalty to Don. She eventually becomes Don's secretary when he returns to the agency.