Electromagnetically induced transparency (EIT) arises because of quantum interference between electronic transitions. While the phenomenon is a "gold standard" in atomic quantum optics, it is hard to probe in bulk condensed matter and difficult to control in quantum-confined systems-prerequisites for exploitation in devices. EIT arises in excitonic transitions of single-layer transition-metal dichalcogenide crystals, which, in effect, constitute giant two-dimensional exfoliated molecules. We exploit the characteristic sensitivity of molecules to their immediate dielectric environment to demonstrate how chemical tuning of the exciton resonance over 5% of the exciton energy allows unprecedented control over quantum interference. EIT is probed in second-harmonic generation (SHG) of monolayer WSe2, where it gives rise to resonant suppression of SHG in response to the immediate surrounding. This solid-state solvatochromic effect arises primarily from changes in electronic band gap and exciton binding energy of monolayer WSe2. Surprisingly, the EIT resonance shifts linearly with exciton energy in response to the dielectric nonlocal manipulation. The approach demonstrates that concepts from atomic quantum optics can be ported directly to condensed-phase materials, stimulating synthetic challenges to develop materials to tune quantum-coherent phenomena.