Her heart was broken when she discovered her best friend was double-crossing her behind her back.

double-crossing 是动词 double-cross 的现在分词形式。double-cross 由 double（双重的）和 cross（欺骗、背叛）组成，字面意思是"双重欺骗"，指"两面三刀、暗中背叛"的行为。