Taylor Sutlive scored 12 points to lead Cal Poly (5-6). Four others scored 11 points apiece for the Mustangs.
The Islanders had a 10-point lead midway through the second half. Reese Morgan and Ridge Shipley made consecutive 3-pointers to spark a 13-1 spurt and Cal Poly led 71-69 with three minutes remaining. Thomas scored nine straight points with a defensive rebound and the Islanders had a 78-74 lead with 20 seconds left.
Morgan missed a 3-point attempt and Douvier made two free throws to seal it.