If you think B's parent should pay an amount agreed by two people without the consent of them or B, plus C should not have to pay anything for driving in the door zone, plus A should not have to pay anything for failing to remind their passengers
to alight on the kerb side (or possibly for parking contraflow and not warning them of passing cars, although I don't recall whether that was confirmed), then I think your morals have gotten twisted: although B made a mistake, it sounds like A and C did too and now have also attempted to stitch up B.