I have talked to plenty of Catholics who say Mary is a co-redeemer and co-mediator.
If that is not in line with Church doctrine, you should get those people IN LINE with what the doctrine really is.
The RCC takes a lot of pride so to speak in having one authoritative doctrine, but that doesn't do much good if you don't enforce it.
>> I have talked to plenty of Catholics who say Mary is a co-redeemer and co-mediator. <<
Mary's acceptance of God's will was, through God's design and not hers, the means through which mankind was redeemed. To express this fact, many Catholics call Mary "co-redemptrix." While what they mean is true and not heresy, the Catholic Church has rejected that title because it may lead to the very confusions that Protestants are wary of.
Likewise, we who do not know Christ in person can know him only through the mediation of the Church, which has given us the sacraments and scripture. (Or direct revelation, but that's certainly an exceptional circumstance, and even then, we'd need scripture to make heads or tails out of it.)
Therefore, the Church, AS THE BODY OF CHRIST, is our co-mediator. And Mary is the embodiment of that Church.