No amount of time could undo the friendship that had been shattered between them.

undo 是动词，由前缀 un-（表示"相反、否定"）和 do（做）组成，字面意思是"取消做过的行为"，引申为"撤销、消除、修复"的意思。在这里指"修复"破裂的友谊关系。