Closed weakly bound bipartite quantum systems typically exhibit relaxation behavior with respect to the smaller subsystem. Here, we investigate a model composed of a finite spin network with one interfacing spin being coupled to a single electromagnetic field mode via the Jaynes-Cummings interaction. The initial pure state of the system can be chosen such that the resulting thermodynamical relaxation process is lasing/nonlasing relaxation or energy back flow from the field mode. We examine the properties of the field mode with quantum optical methods. During the lasing process, the field mode is in a phase-diffused Glauber state with no optical coherence. The thermodynamical analysis of our system is consistent with this finding: The total energy exchanged between both subsystems is found to be heat only. Yet the mapping of this function onto a thermodynamic heat engine appears to be of limited value.