Except in rare cases of those given the gift to read souls, we cannot know whether someone else is or is not in a state of grace. It is up to that person to examine his conscience and determine whether he is able to receive Communion. Mortal sin requires grave matter, full knowledge, and full and free consent of the will. If any of these conditions are missing, there is not a mortal sin. In any case, it is not your responsibility to determine for someone else the state of his soul.
