Is it true that after God showed Mary the souls suffering in hell that she begged and pleaded with her Son to have mercy on them and grant them relief from their suffering? In return Jesus grants them relief on every Good Friday?
I don’t know from what private revelation you saw this or if it has been restated here accurately, but I can tell you that souls in hell cannot be saved and cannot have their suffering mitigated by God. They have definitively chosen against God, and so cannot be helped by him or by his saints.
It is possible, though, that souls in purgatory might have their sufferings mitigated through the prayers of the Blessed Virgin Mary. If that is what the revelation said, then, yes, such an occurrence is possible. Whether the Church has approved of that particular revelation, I could not say without more information.