$140 is a lot to pay for a meal, so I don't really understand the "single mom" excuse. A lot of people who are "money-challenged" would keep their restaurant bill a whole lot lower than that.
BUT (and maybe this might seem like a stretch in the "give the benefit of doubt" department)....
Who knows if someone else wanted her to have a special meal out and let her use their card to cover it (maybe a birthday or graduation meal, etc.). Also, how many people ate? Was she covering someone-special's celebration of some sort? Her parents anniversary dinner? A child's birthday party? Or maybe she underestimated what the bill would come to and (as I think someone else said) realized she didn't have room on the card for a tip that size.
20% of $140 is close to $30. I can see how she may have wanted to cover a special meal but maybe couldn't cover the tip. I can see how, too, she may have thought that leaving a small tip might look as if she didn't like the meal. Or, maybe she's someone who has been a waitress, herself, and who thought she'd be OK with it if a single mother did the same thing. People often do the thing, "If it were me I would understand", because we often tend to think that others think the way we do.
Either way, I know it would be crummy for the waitress; but I'd consider it one of those crummy things that the person may/may not have had good reason for; and I'd assume the best about the person and the situation. Maybe I'd be wrong, but I think it's better to give benefit of doubt to someone who deserves it than not to give benefit of doubt to someone who does.