Yes, but according to a nurse I know, there would be even less opportunity for her to steal drugs - and the nurse I know has never heard of ambien for pre-op. When I've been given a 'calm down' dose pre-op, it has always been through an IV.
If she's on the floor, she can keep a drug she's supposed to administer orally to a patient and when the patient complains about pain or lack of sleep, the assumption will be that the dosage was insufficient. If she withholds the relatively minor pre-op dose meant to calm someone and that someone starts freaking out going into the OR and it happens more than once, it's more likely to be noticed than with patients on the floor who are being treated by different nurses over multiple shifts.
I think KK knew it wouldn't work anyway but just went along with PF because it was easier than telling him that as a nurse she really didn't have easy access to fatal doses of potentially lethal drugs and that even if she did, it wasn't going to be that easy to administer them via a cup of coffee, where the taste might be a tip-off and it would be a challenge to maintain equal distribution of the drug throughout the drink.
Besides, in this instance, I think she wanted an excuse to see KB for herself, up close and personal.
Keep in mind, PF also told KK, a nurse, that a single blow to the back of the head would be sufficient to kill KB with a minimum of blood. You don't have to be a nurse to know that 1) skulls can be hard to crack and 2) scalp wounds, even relatively minor ones, are notoriously messy - as he apparently found out. Yet there's no evidence that KK tried to explain to PF that he was wrong on these points, and as a nurse, she certainly must have known better.