If your friend is drinking coke, or another dark drink, and they leave the table briefly, decant their drink into another container. Then fill their glass with some water, and enough soy sauce to make it look like the other drink. You could also add some of their previous drink for the carbonated "fizzy" effect.

When they return to take a drink it will taste disgusting, and they'll need to wash out their mouth, so give them back their original drink!