We have a Greek Orthodox family that occasionally attend Divine Liturgy at our church. Our missalettes give the guidelines for reception of communion indicating the Catholic Church's welcome to the Orthodox to receive communion. This family, from Jordan, goes up for communion. Perhaps it's cultural?
Or maybe a difference between the Russian and Greek Orthodox churches? Also, I'm wondering if it may be that there is no Greek Orthodox church easily accessible to the family you know. Maybe it's ok in special circumstances? Sort of like we Catholics can go to an Orthodox priest for Confession if we believe ourselves in mortal sin and no Catholic priest is available. I really don't know.