Is it permissible to review and amend someone else's university thesis for a fee, knowing that the amendments may be substantial and that the thesis is part of the graduation requirements, but does not affect the owner's career path?

The questioner did well in refusing to write the university thesis for that person because it involves deception and betrayal. As for reviewing and editing the research: it might be deception or permissible assistance. Whatever relates to the core of the research and its scientific value falls under the ruling of writing [the thesis], while whatever is superficial and marginal, or merely advice and guidance to the researcher, falls under the category of assistance. Sheikh Ibn Uthaymeen warned against preparing research for others and said: There is no harm in seeking assistance from others, but not to the extent that the entire thesis is the work of someone else.