  A simple site-diluted classical XY model is proposed to study the magnetic
properties of the ferromagnetic polycrystalline
Fe[Se$_2$CN(C$_2$H$_5$)$_2$]$_2$Cl diluted with diamagnetic
Zn[S$_2$CN(C$_2$H$_5$)$_2$]$_2$. An extra superexchange interaction is assumed
between next-nearest-neighbors on a simple cubic lattice, which are induced by
a diluted ion. The critical properties are obtained by Monte Carlo simulations
using a hybrid algorithm, single histograms procedures and finite-size scaling
techniques. Quite good fits to the experimental results of the ordering
temperature are obtained, namely its much less rapidly decreasing with dilution
than predicted by the standard diluted 3$d$ XY model, and the change in
curvature around $86\%$ of the magnetic Fe[Se$_2$CN(C$_2$H$_5$)$_2$]$_2$Cl
compound.
