It depends on whether or not your friend is a Catholic who is disposed to accepting the Church’s authority on matters such as this. If he is, I recommend getting information for him on Fatima from the Blue Army of Our Lady of Fatima. If though you are dealing with an agnostic or atheist who thinks all reports of supernatural activity are hoaxes, you are first going to have to get him disposed to accepting the possibility of miracles. I recommend C. S. Lewis’s book Miracles. You might also refer him to the novel The Song of Bernadette by Franz Werfel, a Czech Jew who wrote about the story of St. Bernadette and Our Lady of Lourdes in thanksgiving for his escape from the Nazis. The movie version of the novel opens with this famous quote that could be applied to all such miraculous interventions:
To those who believe in God, no explanation is necessary. To those who do not believe in God, no explanation is possible.
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