© 2021 American Physical Society. The $π$∗-band renormalization of Li-doped quasifreestanding graphene has been investigated by means of isotope (C13) substitution and angle-resolved photoemission spectroscopy. The well documented sudden slope change (known as "kink") located at 169 meV from the Fermi level in the graphene made of C12 atoms shifts to 162 meV once the carbon monolayer is composed by C13 isotope. Such an energy shift is in excellent agreement with the expected softening of the phonon energy distribution due to the isotope substitution and provides, therefore, an indisputable experimental proof of the electron-phonon coupling origin of this well known many-body feature in the electronic structure of graphene.