I don't entirely disagree but all exams are problematic. We don't have the technology to look into a person's mind and see what they know. An exam is an imperfect data point.

Ask the student to come to the exam and write something new, which is similar to what they've been working on at home but not the same. You can even let them bring what they've done at home for reference, which will help if they actually understand what they've produced to date.