She writes:

How would you explain Co-Redemptrix? I have taken a stab at it when an aggressively anti-Catholic person cat & moused me about it, but any help is always appreciated.

I explain it this way in my book Mary, Mother of the Son:

Another title sometimes used to honor Mary is “Co-redemptrix.” It’s not an official title. It’s just an expression of piety among some Catholics. It affords a fairly typical example of the way in which the Church mulls things over for a long time (usually centuries) before it makes any hard and fast decisions. At present, the Church doesn’t condemn the title, but it doesn’t encourage it either. A few years ago, Pope Benedict XVI (then Joseph Cardinal Ratzinger) was asked about the many petitions Rome had received asking that Mary be formally declared “Co-redemptrix.” He replied:

I do not think there will be any compliance with this demand, which in the meantime is being supported by several million people, within the foreseeable future. The response of the Sacred Congregation for the Doctrine of the Faith is, broadly, that what is signified by this is already better expressed in other titles of Mary, while the formula “Co-redemptrix” departs to too great an extent from the language of Scripture and of the Fathers and therefore gives rise to misunderstandings. . . . A correct intention is being expressed in the wrong way. For matters of faith, continuity of terminology with the language of Scripture and that of the Fathers is itself an essential element; it is improper simply to manipulate language.(Joseph Cardinal Ratzinger, God and the World: A Conversation with Peter Seewald (San Francisco: Ignatius, 2002), 306.)

So does this mean that the Church condemns those who honor her with this title? No. It just means that Pope Benedict was (rightly) worried that non-Catholics will not understand the “correct intention” behind the title. The title remains for the foreseeable future something Catholics may use if it matters to them (providing they rightly understand what it means), but it’s not found in the Church’s liturgy or dogma.

That said, it’s worth asking what “correct intention” lies behind the title. When we do, we discover a truth similar to the one behind the honorific “Mediatrix.” Though Mary did not die for our sins, her sufferings were joined to those of Jesus, for the good of the Church. That’s not because she’s a goddess. That’s because the innocent sufferings of every Christian in the world are joined to Jesus’ sufferings for the good of the Church. That’s solidly biblical teaching. It’s why Paul could write, “I rejoice in my sufferings for your sake, and in my flesh I complete what is lacking in Christ’s afflictions for the sake of his body, that is, the church” (Col. 1:24). It’s why the Catholic faith offers such profound consolation to those who suffer innocently. Because Jesus has joined himself to us in our pain, our pains are joined with his. Our suffering is not meaningless garbage that happens for no purpose and does no one any good. Rather, our pain, joined with Jesus on the Cross, has value for his body, the Church, and makes us participants in the redemption of the world.

This is supremely seen in Mary’s endurance of her suffering. For there are two kinds of agony: the agony we feel for ourselves and the agony we feel for another. Jesus felt all the terror of mortal flesh when he contemplated the fate that was snaking toward him as the little trail of torches wended its way across the Kidron Valley and up the slope of the Mount of Olives on Holy Thursday evening. He sweated blood and begged to be spared. Three times he pleaded with his Father to let the cup pass from him. But it could not pass. In that hour, his disciples slept and he was completely alone.

Except for one kindred spirit. We do not know where Mary was at this time. The Gospels are silent. But we know ordinary human at this time. The Gospels are silent. But we know ordinary human experience. We know the anguish of a mother who begs God that her baby be spared the ravages of cancer and that she suffer in her child’s place. We know of parents who drown attempting to save their children. We know of parents who push their children out of the way of oncoming cars and are killed or crippled saving them. We know the agonies of parents bereft of their sons and daughters by drunk drivers, or school violence, or the thousand idiot havocs the world wreaks on our lives. We know how powerfully their hearts cry out like David’s and say, “Would I had died instead of you!” (2 Sam. 18:33). And because of this we know that Mary could not have contemplated the terrible agonies Jesus was about to face without wishing with all her heart that she could take the blows rather than him. Jesus’ cup was to endure hanging upon the Cross. Mary’s cup was to endure not hanging upon the Cross.