inform against/on someone
phrasal verb with inform verb [ T ] uk/ɪnˈfɔːm/ us/ɪnˈfɔːrm/
（向警方）告发（某人）
If you inform on/against someone, you give the police information, usually secretly, about that person, showing that he or she has done something wrong.
The terrorists said that anyone caught informing on them would be killed. 恐怖分子说，举报他们的人一经发现将性命不保。