Put me down in the "if you can't afford to tip properly, don't eat out" column.
Giving less than 10% if the service is adequate is simply unacceptable. My base tip starts in the 15-20% range (I tend to round up to the next dollar anyway.) It can go up if you are just crazy awesome, or down if you are terrible.
I have never just not tipped because I couldn't afford it. I have
left meager tips or not tipped at all when I got bad service. I usually complain to the manager when that happens, too. Fortunately, it is not all that often.
As for the example in the OP... that's just unreal. I sure hope it is the exception. If someone can spend over $100 a meal, they can damn well tip for it properly. "Single mom," my ass.