The phrase "informs on" typically refers to the act of reporting someone's wrongdoing or illegal activities to authorities. However, there are several synonyms that can be used instead, such as "reports", "tattles", "snitches", "rats out", "squeals", "betray", "reveals" or "discloses". Although these words have slightly different connotations, they all imply informing someone else about something that was otherwise unknown or kept secret. Depending on the context, some of these synonyms might be more appropriate than others, but they all serve the same purpose of providing information to others.