As a licensed contractor, you are always subject to scrutiny of people you do work for when problems arise. The prior customer who is claiming water intrusion into his or her attic could very well file an action against you claiming that your chimney repair was defective and allowed water intrusion into the home causing damages and requiring repairs.Whether a lawsuit is filed and is successful against you is another matter. From a technical sense the prior customer does not seem to have a valid claim against you since the leak is the result of faulty flashing if you did not have anything to do with the flashing from the prior work order. Hopefully you took photgraphs of what you saw at the site when you were last there and kept them for future use.It is a good sign that the former customer is calling you leaving messages on your phone and not a lawyer sending you letters.