Along similar lines to what has already been said...
When talking to your Protestant friends, you might take a similar tactic as Christ took, and respond with a question, such as:
Who raised Tabitha from the dead? (Acts 9:40)
Who healed all the people on the island of Melita? (Acts 28:9)
Who drove the demons out of the people of Jerusalem? (Acts 5:16)
It was the Apostles, or more generally, righteous human people. Yes, ultimately Christ did these things through them, but we do not adamantly draw thick lines between the two. Because as we grow in holiness, we take on Christ and His life begins to live within us.
The life of Christ transforms us as people, and it transforms matter itself: people were healed by St. Paul's handkerchief, and even St. Peter's shadow passing over them.
Or to say it another way: we don't become mere channels of Christ's power, rather Christ's power becomes an integral part of our being. This is basically what Theosis is all about. And the Virgin Mary has the greatest part of that power possible, because she is the Mother of God. The Mother of God does not save us, rather Christ who lives within her saves us.
When a person becomes God by grace (not in essence, but by grace—theosis), it becomes hard to draw a line between "Mary saves us" and "God saves us". Mary has so perfectly submitted and conformed to God's will in every way, that it's impossible to differentiate the two.
Mary is not the Divine Essence of God. But through grace she has become so much like God that we certainly must be careful where we draw our lines, lest we deny the ultimate conclusion of Salvation.
(I wouldn't generally go into this much detail with a belligerent Protestant, just because it can be scandalous without the proper framework of understanding.)