This paper proposes a coordinated control framework that establishes synergy between grid-forming battery energy storage systems and demand side management, to improve both dynamic stability and resource endurance in hybrid island electric systems. The grid-forming battery delivers fast active power support, while demand side management provides slower but sustained flexibility, compensating persistent imbalances and preserving battery headroom. A hierarchical control strategy based on time-scale separation and supervisory coordination is developed, where demand modulation is driven by the sustained battery effort and its state of charge. The approach is validated through a 24-hour dynamic simulation of the Lampedusa island power system. Results show improved frequency performance and a significant mitigation of battery state-of-charge excursions, demonstrating that coordinated demand flexibility effectively acts as a virtual energy buffer that extends grid-forming battery capabilities.