While tending the wounded soldier, Dr. Sadao felt nice because he felt a sense of contentment. He knew that he was dedicating himself towards his duty. As a good doctor he always addressed his patients as friend and so while he was operating the soldier, he called him a friend.
These words were ironical to a certain extent because the soldier belonged to the enemy nation. He was fighting against Dr. Sadao's nation. Moreover, being a loyal citizen, Dr. Sadao had thought of handing the wounding soldier to his country.