Meet With Student
University policy requires that before you file a charge of plagiarism, you offer the student an opportunity to discuss the situation. The discussion may take place over email, but it is often worthwhile at least from a pedagogical standpoint to have a face-to-face meeting. This is one of the best ways to distinguish intentional from unintentional plagiarism.
If you decide to pursue a charge after this meeting, warn the student that they cannot withdraw from the course to escape sanctions.
If it is before the midsemester date, the student may be able to secure a signature from a dean in order to withdraw from the course without the professor's permission. The dean has no way of knowing that the student is suspected of plagiarism, and thus has no reason not to sign the form.
University policy does not, however, allow a student to escape a charge of plagiarism in this manner. If the faculty decides to impose failure in the course as a sanction and the charge is upheld, the W will be changed to an F.