Oh thats such a shame. With the fight we are battling pit bull owners really need to know that their dogs actions reflects upon all pit bull owners. While it dosnt seem to be a problem now, ti can potentially get worse. I would approach them from the angle that you just learned about spontaneous attacks that occur to pit bulls from people who fear or hate the dogs.
In the past year two pit bulls have been hung in ther own back yard by a crazed neighbor, several have been set on fire, adn just this week one was decapitated and left on a car hood. Not ot mention plenty of people steal pit bulls to use them as fighting practice for their terrible purposes. Maybe that will scare her into keeping her dog safe.