In order to answer this question a little of the background on the Christian Science sect is necessary. It was founded by Mary Baker Eddy (1821-1910), whose teachings are set forth in the book Science and Health with Key to the Scriptures, first published in 1875. The book describes a belief system Eddy claims to have "discovered" in 1866.
Throughout her early life, Eddy suffered from various emotional and physical illnesses. As a result she developed a morbid fear of the medical profession. She began to view the physical world as an illusion and maintained that the only reality was the spiritual world--possibly as a defense mechanism to deal with the difficulties of her sicknesses.
To Eddy, truth (which she calls "the divine Principle" or "divine Life, Truth, and Love") is a healing spiritual force. She believed that Jesus came to enlighten humanity regarding this truth: "Jesus taught the way of Life by demonstration, that we may understand how this divine Principle heals the sick, casts out error, and triumphs over death" (Science and Health [1971 ed.], 25). The material or physical world, she maintained, is illusory, a product of a wrong perception of our true spiritual nature.
According to Christian Science, things like sickness, suffering, pain, and sin have no objective reality. So when humans experience these things, they are guilty of misperceptions-what Christian Scientists call "error." Christ came to provide spiritual and physical healing by correcting our wrong perceptions. For the Christian Scientist, Christ's passion and death were ways he demonstrated his triumph over wrong thinking rather than being a physical reality he endured: "Jesus bore our infirmities; he knew the error of mortal belief, and 'with his stripes [the rejection of error] we are healed'" (Science and Health, 20; brackets in original).
Of course, the problem here is that Scripture--as well as human experience--paints a radically different picture. Christ did not come to destroy mere "illusion" or "error." He came to destroy the objective reality of sin and its tangible results: pain, sickness, suffering, and physical death itself. This is why Paul says, "If Christ was not raised, your faith is worthless. You are still in your sins" (1 Cor 15:17). There is not even a hint here of something illusory or immaterial. Christ's death and resurrection were objectively real, historical events which triumphed over humanity's actual sins.
So for your friend to consider a visit to the doctor, he would be guilty of not having applied the "Truth" or "divine Principle" which Christ taught. In his mind he is not experiencing an objective illness but rather "error," or a mistaken perception that he is sick. Therefore, going to a doctor in search of a physical cure would be diametrically opposed to his belief system and would only fuel the fire of his "error."