If his friend is busy he will wait quietly at the door until his friend looks up and notices him, and he will give an exaggerated smile, wave, and say hello. If his friend isn't available, there's no harm done. "He (or she) is busy. It wouldn't be right to disturb his (or her) work."
朋友也許正在忙，於是他在門口靜靜的等，等得到對方抬起頭來發現他，他就誇張地揚起笑臉，打一聲招呼；等不到也無妨：「他（她）正在忙，害他（她）工作做錯了總不好吧！」