How should we best deal with our Protestant friends who attend mass for wedding or funerals and go up to receive communion?
If you have the opportunity to speak with your family member before Mass, you could start the conversation by asking whether they’ve ever attended a Catholic Mass before. If they say yes, then you can say, “Oh, so you know what to do at Communion time, right?” You want to give them the benefit of doubt that they know to remain seated. If they say, “Yeah, I go up and receive the host,” then you can begin to explain to them why the Church invites them to remain seated and join the congregation in silent prayer. If the person doesn’t know what to do, then you can proceed to explain to them the proper action.
If the person has never attended a Catholic Mass, then you can say, “I’d like to go over a few things with you about Communion so you’re not totally in the dark.” Once you start the conversation, then you can proceed to invite them to remain seated at the time of Holy Communion and ask the person to join the community in silent prayer.
For some brief explanations on why non-Catholics are not permitted to receive Holy Communion in ordinary circumstances, check out these articles: