So, Mary as Co-Redemptrix.
First off, this was the Papal Theologian during the Pontificate of JP II. You can read a copy of his address here:
Next, the title of what Co-Redemptrix really means. Does it mean? Does it mean that Mary contributed in some way to the redemption of humanity, taking away from Christ's redeeming act? No. From what I have studied, Mary as Co-Redemptrix is as follows. When we say that she is Co-Redeemer, we are stating that she cooperated in the most perfect way with Christ's saving act. She fully and freely cooperated in God's redeeming plan. She said "yes" every step of the way.
John Paul II says the following in 1995.
In other words, the council is stating Mary's role as Co-Redemptrix without using the actual term. Furthermore, JP II has used the term itself in many Papal Audiences and such such as the following (there's more, but I'll only put 2).
During the Council sessions, many Fathers wished further to enrich Marian doctrine with other statements on Mary's role in the work of salvation. The particular context in which Vatican II's Mariological debate took place did not allow these wishes, although substantial and widespread, to be accepted, but the Council's entire discussion of Mary remains vigorous and balanced, and the topics themselves, though not fully defined, received significant attention in the overall treatment.
Thus, the hesitation of some Fathers regarding the title of Mediatrix did not prevent the Council from using this title once, and from stating in other terms Mary's mediating role from her consent to the Angel's message to her motherhood in the order of grace (cf. Lumen gentium, n. 62). Furthermore, the Council asserts her co-operation "in a wholly singular way" in the work of restoring supernatural life to souls (ibid., n. 61).
To Our Lady—the Coredemptrix—St. Charles turned with singularly revealing accents. Commenting on the loss of the twelve-year-old Jesus in the Temple, he reconstructed the interior dialogue that could have run between the Mother and the Son, and he added, "You will endure much greater sorrows, O blessed Mother, and you will continue to live; but life will be for you a thousand times more bitter than death. You will see your innocent Son handed over into the hands of sinners ... You will see him brutally crucified between thieves; you will see his holy side pierced by the cruel thrust of a lance; finally, you will see the blood that you gave him spilling. And nevertheless you will not be able to die!" (From the homily delivered in the Cathedral of Milan the Sunday after the Epiphany, 1584)
Mary goes before us and accompanies us. The silent journey that begins with her Immaculate Conception and passes through the "yes" of Nazareth, which makes her the Mother of God, finds on Calvary a particularly important moment. There also, accepting and assisting at the sacrifice of her son, Mary is the dawn of Redemption; ... Crucified spiritually with her crucified son (cf. Gal. 2:20), she contemplated with heroic love the death of her God, she "lovingly consented to the immolation of this Victim which she herself had brought forth" (Lumen Gentium, 58). ...
In fact, at Calvary she united herself with the sacrifice of her Son that led to the foundation of the Church; her maternal heart shared to the very depths the will of Christ "to gather into one all the dispersed children of God" (Jn. 11:52). Having suffered for the Church, Mary deserved to become the Mother of all the disciples of her Son, the Mother of their unity. ...
The Gospels do not tell us of an appearance of the risen Christ to Mary. Nevertheless, as she was in a special way close to the Cross of her Son, she also had to have a privileged experience of his Resurrection. In fact, Mary's role as Coredemptrix did not cease with the glorification of her Son.
I hope this helps the person who was inquiring. Please leave comments and let me know what you think.