what should I do if I repeatedly observe another sitter (who passed your background check) engaging in illegal/illicit activities in the presence of pets?

Over the last year I witnessed my ex-roomate, who is a current sitter, use various illegal substances around the dogs she cared for in her home. Otherwise she is a great sitter, but my concern for the health of the animals has outweighed my loyalty as a friend to her. I would prefer that no authorities are involved, but something should be done. Whenever I brought up the subject, she assured me that dogs were not affected by her using it. I know this to be untrue. This eventually led me and my boyfriend to move out, but as an animal lover, I'm still concerned about the situation and thought to inform you.