Thank you for asking what I believe rather than, like others have, assume it and then continue to misrepresent me after I objected to the assumption made.
The word “co,” or the prefix “co-” in English can imply equality, and so that’s why it sounds that way. The prefix “co-” comes from the Latin preposition cum, and cum does not necessarily mean “equal.” It can just mean “with,” and that’s the sense that’s in play when the Blessed Virgin Mary is under discussion with respect to the Redeemer. So She’s not equal to Jesus, but She is a woman who cooperates with the Redeemer. She cooperates with God’s plan and Her son’s role in it.
So She, for example, when the angel Gabriel appears to Her, She agrees to become the Mother of the Messiah, the Redeemer, and thus—you know, She’s even told “He will save his people from their sins”—and so She willingly cooperates with the Redeemer, and in that sense she could be described as someone who works with the Redeemer in a unique way, by being his Mother, and you could legitimately use the title for Her in that sense. She also underwent suffering in a different way, as Simeon foretold.
You could also use it for other other people in other senses. All Christians who are doing their job as Christians are cooperating with the Redeemer in one way or another by living a Christian life and setting an example for other people and evangelizing them and telling them about Jesus and doing corporal and spiritual works of mercy. Also, by offering up our own sufferings to God for others, no matter how great, will always be little compared to Jesus's, can be done so for others as well, because all suffering can be redemptive.
All of those things, we cooperate with the Redeemer and his plan. So there’s a sense in which all of us are co-redeemers in this little bitty way.
And so if that’s all you mean, is: “works with the Redeemer in some way,” well then you can use this language regarding the Blessed Virgin Mary, and even with other people.
Yes and those who do have reasons for saying that.