Think of your stomach like the drain in your sink.
If you drink water, it goes through fast and quick.
If you eat food it sits (which is what makes you feel full) until it digests... slowly.
If you drink WHILE (or while there is/or shortly thereafter WILL be) you eat, it flushes the food that is making you feel full through your system quickly so you don't feel satiety as long.
This does 2 things... it empties your stomach making that feeling of fullness not last as long AND it pushes the food through your entire system too fast which means you don't get the nutrients you need from the food you just ate.
And, if you don't believe ME... watch it with him!
I think everyone. I mean EVERYONE... pre/non/post sleeve should really adopt this practise of not eating while/before/during eating. It helps us not eat as much... which of course is what food establishments WANT us to do!