AFAIK, all prescriptions are considered "controlled substances" but some classes (usually opioids, anesthetic agents, anxiolytics, etc.) have stricter regulations regarding their sale and use. Still, in most places it is probably illegal to steal someone else's prescription and use it, but I imagine stealing something like an inhaler wouldn't have consequences as severe as if you stole something like valium or oxycotin. If I were her, I'd still contact the police and they might give her nephew a talking to, even if charges were not filed.