If she says she is “Christian,” she is Protestant. Go to
Catholic.com and read through the articles in their “Library,” about Mary, Ark of the Covenant, about the Intercession of Saints, about the Real Presence, about Faith Alone and Scripture Alone, about Tradition, the Magesterium and the Papacy, all the key issues that create controversy. Purgatory too, probably, and Confession. They have excellent tracts and articles on all of those teachings. You should also look up the Indulgences and read about them online, perhaps from the Catholic Encyclopedia. Protestants often have a lot of misconceptions about that history.
Prayer to Jesus through the Virgin Mary is the key, though, to your evangelistic effort. Simply pray and keep praying, especially the Rosary, and offer Masses for her and maybe novenas. Invoke the aid of great saintly missionaries like St. Therese of Lisieux, St. Francis de Sales, St. Francis Xavier, St. Vincent Ferrer, St. Paul the Apostle, St. Ignatius of Loyola, St. Anthony of Padua (Hammer of Heretics), and Saint Dominic. These heroes of the faith will surely be a marvelous help to you if you pray to them consistently for your friend. Prayer and sacrifice are the keys of victory. The Virgin Mary, the Mediatrix of All Graces, will help you in everything.
Prayer is something you can’t measure with your eyes and it might look like it’s not having an affect for a long time, but God hears and sees our faithful perseverance. It is the front line of our spiritual battle, not the second. Prayer is everything. Without prayer we can do nothing. Get informed through the articles I suggested, and possibly some good apologetics books defending the Catholic faith. Then, when the Lord opens the door for a conversation relating to Catholicism to take place, you will be there for your friend.
God bless you on this enterprise! It is a truly wonderful thing you are planning to do. Praise God for your spirit of service to His Kingdom

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