They felt betrayed when their oldest friend turned his back on them in their time of need.

turned 是动词 turn 的过去式，基本意思是"转动、转向"。 his 是代词 he 的所有格形式，意思是"他的"。 back 在这里指"背部"，引申为"背后"的意思。 on 是介词，表示"在...上"或"针对"。 turn one's back on 是一个固定短语，字面意思是"把背转向某人"，引申为"背弃、抛弃"的意思。