I would not frame this as your disbelief versus her gullibility. Scripture tells us plainly to “test the spirits” (1 John 4:1), so claims of hearing God must never receive automatic acceptance simply because the person appears sincere or devout. I have no biblical basis for affirming that Joan of Arc actually heard the voice of God. Sincerity cannot authenticate revelation. People can sincerely misunderstand experiences, impressions, dreams, or even their own thoughts. Scripture remains the objective standard by which every spiritual claim must be tested. God has spoken definitively in His Son, and we possess the sufficient written Word that equips us “for every good work” (Hebrews 1:1–2; 2 Timothy 3:16–17). That means we should be extremely cautious whenever someone places private voices or revelations alongside the authority of Scripture. So I would treat her kindly, but I would keep bringing the conversation back to this question: Where has God actually promised us that He speaks with binding authority today? Our confidence should rest in what He has written, because there we know with certainty that God has spoken.