1.) How would you add to this customer's soda? Syrup box bags that mixes with the tank behind the concession? Well, in that case you contaminate all who orders drinks, not just a particular customer. Why wouldn't management just do it themselves and leave any unnecessary people who could incriminate them later out of the picture?
2.) In front of the customer? Ridiculous; it open and obvious. At all points, the customer can see you serve the drink from the tap. Even if one could, how would the worker know which customer's drink to spike?
3.) Why wouldn't this person call the police right away? Especially after getting fired for attempting to pollute a food source from an unknown substance. Highly illegal. This person had the moral aptitude to resist, but failed to inform authorities immediately for this attempted sabotage? Odd.
4.) No, lets go a lab and find out what's in the mystery liquid. Sure.