I respectively suggest that that the words mediator and prayer have two separate meanings. From the American Heritage Dictionary "Mediator" One that mediates;especially, a person who serves as an intermediary to reconcile differences."Prayer"A reverent petition made to a deity or other object of worship".
My sole advocate with the Father is the Lord Jesus Christ. On the other hand in the book of Proverbs 15:29 we read "The Lord is far from the wicked: but he heareth the prayer of the righteous". It was the prayer of a Roman Catholic friend at a critical time in my life along with others that made a great difference in my spiritual journey. While he prayed for me he was not the mediator or advocate.
Brother-in-Christ
And that is of course true. But rather than dispute meanings here, let me simply say that when I pray to Mary (and I have done so usually in addition to direct prayer to Christ), what I am asking for is that she join her very powerful voice to mine: "pray for us who have recourse to thee".
That quote from Proverbs is actually one of the verses we cite for the idea of intercessory prayer. I.e. if the prayer of a righteous availeth much before God, then even more so those Christians that are purified in heaven already and have no stain of sin attached to them. I realized that you may not believe that earthly Church can communicate with the Church in heaven. But supposing you did (and we do!), it would be a natural extension of asking folks to pray for you.
True, your friend was an intercessor, which is how Catholics regard Mary when we "pray to her".