Student dissatisfied with exam grade: what to watch out for?
By asking for an exam, the student asked you for a fair, honest, informed evaluation of their mastery of the course material.

If you believe that the exam grade fairly and accurately reflects the student’s mastery of the course material, say that, and explain why.  Be respectful and honest.  Stick to your guns.

In particular, you should have some record of your announcements of both the contents of the exam (“How was I supposed to know this?”) and your expectations for certain grades (“Why did I get this grade?”)—the course syllabus, or a handout, or email to the students.  You should have sources/proofs for the correct answer for each question (“Why is this wrong?”), rubrics for awarding partial credit for partially correct or suboptimal answers (“Why did I get so few points?”), and well-reasoned rules for converting raw exam scores to reported grades.

If you do not believe that the exam grade fairly and accurately reflects the student’s mastery of the material, then you need to reevaluate the exam for all students. You’ve already done this by throwing out one question and curving the scores.