EDIT: In the pilot contract I worked under we had a provision that a pilot would not be required to pay for any equipment he/she may damage in the conduct of his/her duties. I don't see that provision in the linked United FA CBA. So, in theory at least, the company could bill the FA for the $20,000 plus expense of restoring the slide.
Even if that provision was in the contract, I can't see how unnecessarily banging off a slide in order to leave the aircraft, when no emergency exists, rather than using the correct exit and air bridge, can even remotely be classified as 'in the conduct of her duties'.