I was like... what? How did she know this? Now I know. Moms get up before anyone else does. That must have been how my mother witnessed this while no one else did.
I kept it in the back of my mind for the next incident. It was inevitable. So I stood up on the bus, faced Kristen with boldness and said, "Well my mom says that she sees your mom jogs outside every morning wearing only a bra and panties and pantyhose!" Immediately, she started crying like a baby. Her toughness went away quickly, and she sobbed and sobbed. I didn't know whether to feel bad or not. Obviously, Kristen knew her mom did this and was humiliated. Good news is that she was never mean to anyone again.
Lesson learned: Don't make it a habit of going outdoors in your underwear. You'll humiliate your kids.