It is now common courtesy (IMO) to carry a pooper scooper when walking your dog. With all the "appliances" you can buy you don't even have to bend over to pick it up!
I also hate looking over to see someone standing there innocently while their dog takes a dump on someone elses lawn....as if it's ok. Nothing like coming out of your house to find a steaming pile of s*it waiting for you.
Perhaps you should make a sign and post it in the park that politely asks people to pick up their stinky poop, with a note "you know who you are" on the bottom. Maybe she will see it and take the hint. If not, perhaps you could ask her to pick it up without making a scene. Or call your city park maintenance and request a real sign.
Good Luck. I too hate the Poop Leavers.