Well, if you take that verse in a *completely* exclusive sense, then a Christian can't ask another Christian to pray for him. Is that practice ok?
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