Yes, it is forbidden, in the Orthodox Church, to pray together with those of a different faith. The Moscow Patriarchate, by far the largest and most predominant Orthodox Church in the world, has clarified this recently and it is on the conscience of every Orthodox Christian whether Russian Orthodox or not.
But attending some event at a non-Orthodox church is not the same as the sin of praying with the non-Orthodox. I'm sure the fellow who attended his family member's chrismation did not actually say the prayers together with the Roman-Catholic people, or sing along on anything. If so, it was a sin. But doubtless he was simply showing respect for his family, not (God forbid!) seeking some kind of spiritual nourishment from the heretical church.
One can show love, without sacrificing the truth of Christ. Due to heresies having entered in, we cannot all participate in Sacraments together, but we can participate, without asking any blessing or waiting on anyone else, jump in and participate in the Sacrament of Love for one another.
Just because someone does something wrong, or picks up a wrong belief, is not a reason to cease loving him.