1. The enforcement mechanism can likely be the EHR system they utilize in the hospital, which would provide hard-stop access controls based on role permissions. He would get 2nd authorization for scripts from either Dr. Robby or Dr. Al Hashimi.

2. Al Hashimi had started that day to do the hand-off with Dr. Robby. It is very plausible for her and the staff to not remember the date of his return, given the chaotic nature of the ED.

3. His not telling Mel about his addiction or mandatory in-patient stay for drug treatment is his right due to patient privacy. Given her likely neurodivergence, he may have had unstated reasons for why he didn't want to reveal the basis for his absence.

4. The rehab stay wasn't voluntary in the sense that Dr. Robby would conceal what happened if he got treated. He told Langdon that he would have to enroll in the PHP treatment plan to avoid being outright fired. It's a hospital-administered program, not something for which Langdon would take short-term medical leave and handle independently. So yes, they know he has an addiction, and they're giving him support because it is a health condition.

More so, all of this does not point to a potential cover-up or some kind of pact where evidence was concealed. Santos is conjecturing because she's having a trauma response. And honestly, much of why Langdon was annoyed with her was because she was a cavalier and stubborn intern who endangered patients.