Praise be to Allah.
The aim of the research that is required of students in universities is to train them in research skills and test their abilities, and to accustom them to referring to sources, and so on.
The students must do this work themselves, and it is not permissible to seek the help of others with regard to what is central and fundamental to the topic of research, whether that is in return for payment or not.
But if the student seeks the help of someone else with regard to that which is not central and fundamental to the topic of research, there is nothing wrong with that or with other people helping in that regard, because no academic thesis can do without seeking the help of a supervisor, colleague or expert to offer suggestions and advice, and so on.
But if the system of this particular university does not allow that, or the general academic custom in the country where the university is does not allow help at the level mentioned, then it is not permissible to do what the system or custom disallows.
One example of that is if one of the main aims of this research is to train the student in improve his usage of the language and make him more fluent in it, especially if the language of research is not his mother tongue.
What appears to be the case in such situations is that it is not allowed to seek the help of others to edit the thesis or correct its mistakes, because this leads to missing out on an important and valid aim, to which attention must be paid.
Please see also the answer to question no. 104812 .
And Allah knows best.