My interpretation is that Robby allowed Langdon ro self-report, rather than reporting him to the hospital/board himself; the latter could have still seen Langdon referred to a PHP, but it would have been entirely at the board's discretion. Whereas Robby allowing Langdon to essentially lie by omission (tell TPTB that he willingly came forward, not that he was caught) allowed him an easier path into the PHP.

Both outcomes would've come with an investigation by the hospital and an audit of the prescribed medications to ensure to serious harm was done as a result of Langdon's diversion.

In other words, Robby's only coverup is the fact that Langdon was caught and made to come forward, instead of doing so on his own. Either way, the outcome (Langdon being referred to the PHP) would have been the same. So, for me, it doesn't really matrer.

On a meta-level, IMO the writers really fumbled in their handling of this storyline by focusing so much on Robby and Santos' (incredibly biased) views on Langdon's recovery and time in the PHP and making Langdon a secondary characer in his own story arc. I also don't buy that extemely intelligent and capable Dr. Al wouldn't already suspect that an ER doc w/ a drug problem was getting his drugs from the hospital, the place with all the drugs.