Quite frankly, I find doormats like the LW to be so irritating that I have to walk away from the screen just to compose a reply.
My response would have been to ignore the pointed looks the first time, and if the other mom point-blank asked, I would simply say that I had only budgeted to cover my own expenses, and suggest she call her DH or someone else to provide the funds for her to cover her own expenses. Repeat as necessary (since it appears to be a regular occurrence) until she gloms onto some other person who has hopefully learned by the example set to not be a doormat either.
Being thought cheap? That doesn't even make sense to me because LW is covering her expenses. I don't care how small the amount is, I don't cover for casual "friends" (most people seem to use this word far more loosely than I would, I would consider the other mother to be an acquaintance).
POD x million.
My reply to the moocher would have been "oh dear, you'll need to get the manager over so you can work something out with him/her". If the manager or other diner tried to guilt me into paying moocher's tab I'd refuse. Not my problem. That said, if a close friend genuinely forgot their purse/wallet I'd be happy to cover them as I know all my friends would repay it as soon as they had access to their money again (and that has happened twice as we all have brain farts occasionally and both times I was repaid the next day, without having to even ask). I wouldn't pay for an acquaintance.
I sometimes wonder if I've lead a very sheltered life, or maybe I've just been very lucky, but I've never found myself in any of the situations I've read about on ehell or other sites, when it comes to paying for meals. Everyone I know just pays their own way, has sufficient money to cover their bills and we all just get on with it. A lot of the places we go to eat are the sort of venues where payment is required upfront when placing your order at the till, so there's no way for anyone to order $500 worth of stuff and then try and get out of paying. But even at restaurants where a bill is brought to the table after eating, everyone knows what they ordered and pays exactly that (taxes are already included and there's no tipping, so if I order a $9.95 menu item, that's the exact amount I'm required to pay).
I honestly don't understand the mindset of someone who would (A) leave the house to socialise without their purse/wallet and (B) have an expectation (ahead of time) that someone else will pay for them. If anyone can explain that level of arrogance and entitlement to me I'd be genuinely interested. Where I live, it's illegal to drive without your license, which 99.9% of people I know keep in their purse/wallet, along with debit/credit cards, cash, medicare card, etc. I'd also be terrified of being stranded somewhere without any means of payment should something really unexpected happen. So why do they do it?