Plumbing problem caused by block city sewer line

My sister at 760 Florence Street has blocked sewer line caused by a problem on a break,etc on city property.
My contractor, Tony Barrows was told by AJ that we have to prove 100% that the city is at fault. My sister's counselor, Loretta wanted me by posting this if anyone in this area is having similar problems. My sister is 67 and disabled and being left with this problem and no solution /next step from public works about what to do except AJ lack of interest in her problem Anxious and frustrated .   ANy ideas  her sister