Reporting attempt at cheating
If your institution has an honor code or similar description of the requirements for academic conduct, it may prescribe what you must do.  The solicitation of a "ringer" to take the exam may itself be an offense.  If there's a prescribed action, follow it.

If not, consider that an accusation of cheating by the professor necessarily creates an adversarial situation.  If you do report this to the professor, please copy the department head, dean, chairman of the honor council.  Copy at least one other person who can support the professor.  Include a copy of the student's email with your report, and retain your own copy for a very long time.

Do not respond to the student.  Chastisement will not do any good, and while the student is awaiting your reply, he or she isn't looking for someone else to take the exam.