Complex sentence: After their work on "Unfaithful", Mandler became Rihanna's regular collaborator, and worked on most of Rihanna's videos, including "Disturbia" (2008) and "Russian Roulette" (2009).
Split into simpler sentences: After their work on "Unfaithful", Mandler went on to become Rihanna's regular collaborator. Both of them worked on most of Rihanna's further videos including "Disturbia" (2008) and "Russian Roulette" (2009).