Sorry, is this a proof of Flat-Earth, or Ball-Earth? It sounds like a proof of Flat Earth, because if it were a proof of Ball-Earth, you would have used a ball in your experiment, rather than a cup. And trust me, the water leaks out for the ball!
Learn the maths. Understand it. Only then can your disagreement have any validity.
Do the experiment. You would need to record it with a camera, preferable with a high speed one, because the effects will be small and the timeframe as well.
Take a ball, wet it, drop it. Look at the footage you recorded. Does that water drop quicker than the ball? Does the ball drop quicker than the water? Or do both drop at the same speed?
But I guess you refer to the many many Flat Earther "proofs" that pour water over a ball, see it drop to the ground and lament that it doesn't stick to the ball.
Well, do the darned math!. How big is the gravitational force between the ball and the water? How big is the gravitational force between the water and the big rock that we are standing upon? Why do you think you can ignore the pull of the earth on the water, when you are still holding the ball? What do you think is holding the earth, but not the oceans?
It is always the same: you don't understand what you are talking about, you ignore the explanations you get, you deflect to another question, showing that you don't understand that problem either, and never never ever get to explain your own system beyond "magic".
(And you know what? When you pour water over a ball, most of it will run off. But most likely, a tiny film of water will stick to the ball, due to surface tension and electrostatic attraction. In scale, this film is thicker than the oceans on earth. But scale is another of these things that Flat Earthers just choose to ignore.)