If you watched someone poison another's drink, did nothing, & the victim died, what would charges be?

It's not a crime to watch something that only later turns out to have  been a crime, unless you actively participated in its planning or execution.  Similarly, it is generally not illegal to watch a crime in progress and not report it, let alone refuse to try to stop it.

It's not conspiracy unless you agreed with others. It's not aiding or abetting to simply watch it happen; aiding or abetting require proof of some physical action. You're not an accomplice,  before or after the fact for simply keeping your mouth shut.

You never have any legal obligation to speak to the police. Period.  Yes, you may face pressure to assist them, but you don't have to.  Yes, there are a few laws that require a person to report finding a dead body.

Yes.   Conspiracy?   Aiding and abetting a murder?    From a MORAL standpoint,  If you can see something is happening that will harm a person and you do not speak up you are just as guilty. 

yes -- you would be an accessory before the fact of a murder -- which, in my state, is punishable as if you had done the murder yourself

Depends on where.  Aiding and abetting if you didn't know what was being put in the drink.  My neighbor's daughter died of an overdoes of a date rape drug.  You are again confusing TV and real life.

Happens more often than you might think.  (Date Rape drugging)

What you observe, may not be what it is.

You could always interrupt the action, warn the lady...

Keep your mouth shut, "see nothing",  as in mind your own business.  Admit nothing; You don't have to talk to the coppers.

A persecutor (sic) might try "Hindering persecution", "interfering with an investigation."

How would it be proven you saw it?

After all, in today's world, there are plenty of public murders that occur amidst a large number of people, and when asked by police, nobody saw nothing.

It's kind of why murder cases have clearance rates below 25% in the larger cities.  

Nothing. In most situations there is no duty to affirmatively act to prevent another's death. There are exceptions for certain relationships like caregiver/patient, parent/minor child, etc. 

The watcher if caught would be charged as an accessory to murder! just like all the LIEberal left socialist demoCROOK supporters should be charged as an accessory to commit fraud for blindly accepting the fraudulent results of the 2020 election and NOT demanding a FULL fraud investigation!!