Is it permissible for a university student to write a graduation thesis after having prepared the research plan and gathered most of the information, with only its arrangement and formatting remaining?

It is not permissible for someone to do the student's work in research, such as gathering information, formulating it, arranging it, and extracting results. Assistance is permissible in matters unrelated to the core of the research, such as typing, provided the university does not prohibit it.