It's an old story, I'm afraid. Old as Eve and the apple. That snake, the Joker must have promised her some baubles. Led her astray.
12 seconds sound clip from Batman (1966) - He Meets His Match, the Grisly Ghoul (S1E16) TV Series.
You can hear this line at 00:07:30 in the Blu-ray version of the movie.
Quote context
[...]
- Holy Benedict Arnold, Susie!
- Our chief cheerleader. A member of the Joker's criminal gang!
- Steady, Robin. Steady, old chum.
- But what are we gonna do, Batman?
- It's bitter... but it's a break. The first one we've had. We'll have to use it.
- Let's see, today is a school holiday, isn't it?
- Where does that cute little trickster happen to be hanging out?
- I know. The Easy Living Candy Store. It's on East 3rd, two blocks from school.
[...]