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.
For the record, I was present at these RCC services as a passive observer. No participation in any way, and certainly no receiving of Communion. Though family members of the newly-confirmed were invited to receive a blessing without receiving Communion (those who were not receiving, be they RCC or not, were asked to cross their arms over their chests), I chose not to join the queue.
And, Stanley, you have yet to answer my question of the existence or otherwise of mixed marriages in your family.