Is my sister obligated to pay a drug dealer for prescription medicine she purchased on credit?

My sister has finally accepted help with her addiction to prescription pain medicine. Since she has been in recovery i have been receiving harassing calls from the dealer stating that she owes him money. I am aware that this is an illegal issue however i'm curious about what can the family do to stop him from harassing us & also prevent him from trying to do anything to her once she has completed her recovery. My dad & I just would like to put all of this behind us but it's so hard to do when we have been receiving these phone calls or text messages 2-3 days a week. I am ready to get my sister back to living a normal life without the stress of someone like this that can risk her to relapse if this continues once she is back home. I cant get him to give me any information outside of her owing him $100. I have no idea if there is really any truth behind this or not which is one reason why we haven't offered to pay him. What really amazed me was why would you loan someone anything when they don't have a job or any means of repaying. I am so afraid to mention this to my sister at this time because she has been doing so well. I don't want to loose her again.Can anyone please assist me & my father with this issue. We want to handle this in a way that our family will not be in any harm.