As the story of the miraculous image of Our Lady of Las Lajas became better known, skeptics like your friend made the accusation that the Dominican priests who had been evangelizing the region in the 18th century painted the figures of Our Lady and the Christ Child on the rock. These objections about the authenticity of the painting of Our Lady of Las Lajas led to tests on the rocks..
Several bore holes were drilled into the rock to remove small bits of the picture. Geologists who examined the rock noted that there was no paint or other pigment on the rock. The colors making the painting are the natural colors of the rock. No other tests have since been conducted on the rock.
This information comes from the work Historia de Nuestra Señora de Las Lajas (Pasto, Colombia: Tipografía y Litografía Cabrera, 1980).
We would suggest your friend to get this work to learn more about the scientific testing.
TIA correspondence desk