My sisters dog bit someone while on a leash now she is being harrassed

My sister was walking down the hall of her apartment with her small Dachshund on a leash to take her outside, a neighbor walked out to set out their garbage and before she had a chance to say anything he reached down to pet the dog, the dog bit him and now he is threatening her requesting payment for medical bills, and harassing her. He seems to be looking fr a lawsuit and my sister has nothing to give. What are her options and does he have a case.