Ok, deep breath. Bear with me for one second.
Objectively it is sacrilege to receive communion in a state of mortal sin. Agreed.
But if the person doing so doesn’t know it is sacrilege to receive communion in a state of mortal sin, then subjectively they are not guilty of any sin whatsoever.
The intention is very significant. We can’t gloss over it or pretend it makes no difference. Lack of full knowledge or lack of deliberate consent mean no mortal sin, period. That is our faith.
If I run someone over with my car because my breaks suddenly decided not to work, objectively I am culpable of a grave sin, but subjectively I am not guilty of mortal sin because it was accidental.
Concealing mortal sin from the confessor is also very serious because it can invalidate the confession, but there is again the issue of deliberate consent. If a person is deeply ashamed or traumatized or has some other serious reason that keeps them from confessing, that’s one scenario. If the person willfully chooses to hide the sin from the confessor that’s another scenario. But that’s for the confessor to work with, not for us.
The best we can do is encourage our friends, remind them that God already knows our sins, and suggest that perhaps they may write those sins in a piece of paper to hand to the confessor if they are unable to speak up (and then of course destroy the paper).
God is much more gentle with us than we are with one another.