I have had customers "palm" me a very decent cash tip. then pay when the bill comes, the amount of the bill only. I would be mortified if a neighboring table insulted that customer in such a way! What if it was their first time dining in the resataurant, and were taken aback by such rudeness? and therefore (perhaps assuming the outspoken busybody was either a friend of mine or a regular customer) chose not to return?
another possible scenario. My dad took the family to dinner, treated us all. he's a very generous person, but many elderly people don't "get" tipping. they still tip the same amount they did in 1945, no matter the bill.

. so when the bill is settled, I leave extra cash on the table. To a neighboring table, it might look like that is the entire tip. If they brought attentionto it at that time, they would be embarrassing my dad and not doing the server any favors.
I've also been tending bar when a customer sat at a table, so tipped throughout the evening as he purchased drinks. then, as leaving, you get the guy who yells out "hey! aren't you going to tip the bartender?" now
that guy (who probably tipped me more in that one night than the loudmouth has in a month) will probably not return.
the topic of whether to tip or not is, ... well, a whole 'nother topic. (I'm sure you all must now where I stand on that).. but that patron was just rude.
I would be interested to hear if the server heard all this and if so, how did she handle it?