Previously helium atom scattering (HAS) has been shown to probe the electron-phonon interac-tion at conducting crystal surfaces via the temperature dependence of the specular peak intensity. This method is now extended to non-stoichiometric superconductors. The electron-phonon in-teraction, as expressed by the mass-enhancement factor λ is derived from the temperature de-pendence of the diffuse elastic scattering intensity, which specifically depends on the non-stoichiometric component responsible for superconductivity. The measured value of the mass-enhancement factor for Bi2Sr2CaCu2O8+ξ at the optimal doping ξ = 0.16 is λ=0.55±0.08 in good agreement with values of λ recently estimated with other methods. This confirms the relevant role of electron-phonon interaction also in high-temperature non-stoichiometric cuprate superconductors.