He couldn't believe that his best friend had betrayed him.

betrayed 是动词 betray 的过去式，betray 的意思是"背叛、出卖"。这个词由前缀 be-（表示"使..."）和古法语词根 trair（"背叛"）组成，字面意思是"使背叛"。betrayed 表示"已经背叛了"的行为。