Mental status does not determine her eligibility to remain on the court. She is appointed for life. So long as the body is breathing, even if there is no awareness left, she is eligible and the Democrats will probably ensure that she maintains her seat. At this point she probably can’t decide to retire so she will stay on the Court and issue Opinions through the interpretive offices of her staff.
I agree that she can’t be booted but maybe Roberts could decide that voting by the Justices will occur in person and be expressed verbally with no aides allowed in the room?