I have a married couple that are my friends. They have one daughter. Husband was baptised Catholic, but not a practicing one; Wife is not Catholic; daughter attends Catholic school, but not Catholic. They attend a non-denominational church. We all recently attended Mass at a Catholic church near our summer camps. Husband and daughter received Eucharist. Should I talk to them about this and explain why they should not participate in communion? I have a feeling they will be defensive; but perhaps it is I who should be defensive.
These links should be helpful for understanding why non-Catholics should not take Communion at Mass. If you have any further questions or concerns that are not answered by these links, please contact Catholic Answers directly.
*]Who Can Receive Communion?
*]Ecclesia de Eucharistia **(Note especially sections 43-46)