Can any dead person give an act of charity to another that is living? No! Of course not!
But our Lord, Jesus Christ says, "Unless you eat of my body... you do not have LIFE in you." But we are also warned by the Holy Ghost from St. Paul, that those who eat not discerning the Body of Christ bring condemnation on their own souls.
Charity begins with Christ, and the means is via his Sacraments. If we place Christ's Sacraments on par or even below the dignity of good works, then we are inferring or saying (deliberately or unintentionally) that Christ is immanent. Is this dangerous? You better believe it is. It opens the flood gates to all kinds of heresies. No one but someone who is called a "conservative" pope can bring about this type of confusion, because if he were openly considered "modernist" or "liberal", those Catholics who are already lacking knowledge of the some essential theology would be able to more readily detect what is happening and they would refuse it. Would any of you accept if the Pope did away with the Sacraments? He IS Pope, is he not? Of course you would not go for that. Would you accept if he directly stated that works of charity are greater than the Sacraments? No, you wouldn't be fooled by that, if he directly stated that.
Let me share with you, that many bishops and cardinals believe that, and only have Monsignors around them and other folk that also believe that. Are they still bishops and cardinals? Yes, of course, but do they have the God-given authority to say whatever they want? No. Their authority comes from God, and what they say by their words and actions must defend what God has already taught the Church, without confusion, without contradiction of one teaching vs. another. See, heresy is not merely going against the truth, it is taking a truth that is known and bending it disproportionately until it is against other truths. One of the biggest in modernism is immanentism.
This is why we have beautiful cathedrals, statuary, and gold leafed altars, and gold chalices, etc., because it is God who we are celebrating from whom all good things come. It is He who gives us the supernatural life through the SACRAMENTS, and the ability to do the works that He desires as HE wills. Our works are not greater than His at the altar of every Holy Sacrifice of the Mass. That is why we have to be careful to honor our Lord's sacrifice with the greatest of possible dignity we can afford, in order that we not be poor spiritually, and thereby can later bring Christ to others, both in the one Spirit of truth and charity.
Your first sentence is in error.A dead person in purgatory can do nothing to hasten the end of their purgation. But they are still able, as members of the body of Christ, to offer prayers for the living.