  We use exact quantum Monte Carlo simulations to demonstrate that the N\'eel
ground state of an antiferromagnetic SU(2) spin-$\frac{1}{2}$ Heisenberg model
on the honeycomb lattice can be destroyed by a coupling to quantum phonons. We
find a clear first-order transition to a valence-bond-solid state with Kekul\'e
order instead of a deconfined quantum critical point. However, quantum lattice
fluctuations can drive the transition towards weakly first-order, revealing a
tunability of the transition by the retardation of the interaction. In contrast
to the one-dimensional case, our phase diagram in the adiabatic regime is
qualitatively different from the frustrated $J_1$-$J_2$ model. Our results
suggest that a coupling to bond phonons can induce Kekul\'e order in Dirac
systems.
