The doctrine says that formally Mary, the wife of Joseph of Nazareth was the Theotokos, which is Greek for ‘God-bearer’. Does that help your unbelief. After all, this is a doctrine accepted by every Christian branch of my acquaintance, so I’m not sure where your correspondent’s heresy can be said to come from.
Mary was indeed blessed but no more than anyone else who walks in the ways of the Lord.
One would hope they could see this heresy.