  We present a combined experimental and theoretical work that investigates the
magnetic proximity effect at a ferromagnetic-insulator - superconductor (FI-S)
interface. The simulation is based on the boundary condition for diffusive
quasiclassical Greens functions, that accounts for arbitrarily strong
spin-dependent effects and spin-mixing angles. The experimentally found
differential conductance of an EuS-Al heterostructure is compared with a
theoretical calculation. With the assumption of a uniform spin-mixing angle
that depends on the externally applied field, we find good agreement between
theory and experiment. The theory depends only on very few parameters, mostly
specified by the experimental setup. We determine the effective spin of the
interface moments as $ J\approx 0.74\hbar $.
