To exploit a rift to sow discord; to drive a wedge between people by seizing an opportunity.
他们之间本就有一些小摩擦，结果被有心人乘间投隙，关系变得更加复杂了
There were already some minor frictions between them, which were then exploited by an ill-intentioned person to sow discord, making their relationship even more complicated.
Next Idiom