Poor thing. You are right that our kids seem to reserve the worst for us! It's like we're their punchbag. It's because we're safe, imo. Not that knowing that makes it feel any better!
You did a very good thing. Do you think you'll see her again? Maybe invite her round or yes, tell her about mumsnet.
Sounds like she's at the end of her tether. Poor woman. Sadly, kids can sense this - they pick up when you're stressed or upset and it agitates them, making you more stressed...it's a vicious circle.
In theory, there's lots you can do to help her, depending on how she feels about accepting help and support and the level of involvement you are comfortable with. OTOH, of course, you might never bump into her again, but at least you know you did a very good thing. I bet you made a hell of a difference to her day.