We consider the Hamiltonian version of a $\mathcal{PT}$-symmetric lattice that describes dynamics of coupled pendula under a resonant periodic force. Using the asymptotic limit of a weak coupling between the pendula, we prove the nonlinear long-time stability of breathers (time-periodic solutions localized in the lattice) by using the Lyapunov method. Breathers are saddle points of the extended energy function, which are located between the continuous bands of positive and negative energy. Despite not rendering the energy minima, the breathers are shown to admit an approximate Lyapunov function which helps us to estimate evolution of perturbations on a long but finite time interval. The nonlinear stability analysis becomes possible for the $\mathcal{PT}$-symmetric lattice only because of the existence of a Hamiltonian structure.