Complex sentence: Siegel had been a mentor of director Sam Peckinpah, who at this time was having difficulty finding work in the industry due to his troubled film productions, and he gave offered Peckinpah 12 days of second unit directing on the film.
Split into simpler sentences: Siegel had been a mentor of director Sam Peckinpah, who was having difficulty finding work in the movie industry due to his troubled film productions. Don offered Mr. Peckinpah 12 days of second unit directing on the film.