I work as a hospital supervisor. I honestly don't know of any facility where you can't get saline iv flushes. Most nurses have pockets full of them each shift. If you didn't have that for some reason you could get sterile saline and draw it up yourself, also would be easily obtainable.
My only thought here is that the person didn't want to leave the room when administering drugs and so they used tap water as an easily sourced replacement for the drugs they was stealing since there is a sink is every room (at least in most hospital rooms).
The real answer here is that drug addiction overrides rational thought and they either didn't know or didn't care that tap water is not safe at all for iv administration.
We see lots of cases of diversion unfortunately because these drugs are just so damn addictive and there are only so many safe guards you can put in place to protect against it. At the end of the day no matter how many checks there are you will eventually have a clinical staff member drawing up the drugs and administering it. As long as this remains the case you will have people that abuse that weak link in the chain.
I suppose it's good that he wasn't just outright assassinated. Not that this won't occur if putin sees the need for it.
Here's hoping 24 will see putin finally answer for his many crimes and this man set free along with so many others.