Appalling case, I feel so sorry for those women. Egg retrieval without analgesia must be appallingly painful. Add in the themes of ignoring women's pain and downplaying the seriousness of their experience. They must be so traumatised.
Yale behaved appallingly, management cover ups/failure to take responsibility aren't confined to the NHS.
I don't know what the rules and systems around use of opiates, drugs that are controlled drugs in the U.K. , are in the USA, I expect it varies from one state to another for a start. It appeared that the fentanyl was supplied in ampoules like some we have for other IV drugs, with a rubber bung that the needle is passed through, often for multiple patient use, which has a sealed plastic top on when new. This was how suspicions were raised, when an anaesthetist noticed the plastic tops were loose.
Fentanyl (in my corner of the NHS) is in glass vials, so it couldn't be readily replaced with saline in that way. Although if there were sloppy systems, eg two nurses checking the drug but one getting on with other work whilst the first administers the drug, possibly in a different location to where the drugs are stored and drawn up, it would be possible to swap the syringe for one prepared earlier containing saline only.
Sadly drug addiction and misappropriation of controlled drugs does happen, we have had an instance in my hospital earlier this year, a junior anaesthetist was found to be the culprit. At least they were simply misappropriating the drug, not substituting it for something else.