I seldom see a surprise ending that I like, but that's because they're often poorly done.
This one was done pretty well, so the surprise ending worked well for me.
The one thing I found that took me out of the story was
This is a surprise, but not one that should have been there. The opening of the scene strongly suggests (without saying so) that the POV character would not have had the opportunity to do this, and this is information that the reader needs up front. So, you need to come up with some way of telling the reader early about the mickey in the drink.The stuff I had slipped into her drink was strong and working in full effect.
It doesn't have to be stated outright; it might be enough if the POV character could exchange significant glances with the bartender ("a good friend") before approaching the victim. If that seems too subtle, you could put in a little bit about how the POV person had slipped the bartender a packet of powder and a twenty before the target came into the bar, or at least before she sat down.
The only other thing that bothered me was
The diagnosis is an admission of guilt of sorts. Much better something general, like, "Hey, I think this lady's sick; better call the squad.""Call the paramedics. She's having liver failure," I told him.
Other than that, not bad.
Hope this helps,
LC