Check your local laws--animal cruelty laws do include reptiles in many places. However, the more diplomatic approach is to not resort to this until you inform the person on what they need to do to care for the animal properly. If they fail to make the changes immediately, THEN you report them to the law.
If you do a search on 'animal cruelty reptiles' you will see that reptile cruelty cases are prosecuted. You merely need to find an authority knowledgeable enough to recognize mistreatment.
First you need proof of abuse. Hearsay is not proof and inadmissable in court...