I don’t see that it would be a problem in the situation that you mentioned. When the client hired the other designer, she was effectively purchasing the rights to the work-it was a design made for her that she purchased. Therefore, if she asks you to make changes to it, there shouldn’t be an ethical issue. It is not as if you found another designers’ work and copied it or parts of it without her permission–when a designer works for a client or employer, the designs done within the scope of that work would belong to the employer, unless explicitly stated otherwise.