Yes, the police will often not arrest or prosecute a confidential informer while he/she is acting as an informer, even though in the process, he or she is committing crimes--the judgment call is made and the authorities use their discretion to conclude that allowing John/Jane Doe to, say, use or deal drugs, in order to then get evidence to prosecute more drug users or dealers, is a good tradeoff. However, that protection does not extend to anyone else withe C.I., since the whole point of a C.I. is to get information about and arrest the people committing crimes with him or her.If your your friend has been using drugs no matter who is using them with (a "regular" person, a C.I., an undercover copy), she can be arrested and charged.