Well, I assume that he will know he should not go to Holy Communion, as that would be a sacrilege and would damage his soul.
I hope this is not the case. However, if it happens, they should admonish him fraternally not to do so and advise him to speak with the priest. But simply attending Mass, I would say, is something they can encourage, because we don’t know when God will touch his soul.
There may come a moment during Holy Mass when God touches his soul, and he could experience conversion. In this case, we must continue to support him. If he is Catholic, he is obligated to attend Mass; this is his duty. And then, we should pray for his conversion.
Dear visitors of Gloria Dei and all who have already written or who will write a personal message to me. I am grateful for your attention, your kindness, for your spiritual support and especially for your sacrifices and prayers for me. God may reward you!