Complex sentence: Zach's friend, Dean Sampson, Jr. (Paul Walker), disagrees, so they make a bet on whether or not Zack can turn any girl into the Prom Queen within six weeks.
Split into simpler sentences: Zach's friend, Dean Sampson, Jr. (Paul Walker), disagrees. They make a bet on whether or not Zack can turn any girl into the Prom Queen within six weeks.