Her biggest ally is something called The Fair Debt Collection Practices Act. It's a federal law that governs what debt collectors can do. It's easy to get a bill collector to stop calling. All Sheri needs to do is to notify them in writing that she doesn't want to hear from them anymore. She doesn't need to give them a reason.
Certified mail is best.
After the letter is received the collection agency can only contact her to let her know that they won't be calling her again or to inform her of pending legal action. Even if Sheri did still owe the money, she has quite a bit of protection under the law. The same request to stop calling works even if you still owe the debt. Obviously stopping the calls doesn't relieve you of the responsibility of paying your debt. Collection agency tactics are regulated by the Fair Debt Act.