A glass of water means approximately nothing. It isn't even a drop in the bucket compared to just the water that is used to washed down the drain to clean up the things that are dirtied in the process of producing the thing known as "dinner, at a restaurant." It is an even smaller non-drop compared to industrial and agricultural uses of water that a restaurant has no control over at all.

To pretend that providing an unprompted glass of drinking water (or not) makes any significant difference is reprehensibly inept.

(To be clear, I don't think that your description of the reality you observed is bad in any way. The report is fine; it's a good report. The thing you described in that report is simply very ugly, in and of itself.)