Your student should reach out to their counselor and the dean of students. It's not a simple question. You can withdraw at basically anytime but it will stay on the transcript as W/P or W/F. Details are here:
https://go.boarddocs.com/vsba/fairfax/Board.nsf/files/BF2RTL69049F/%24file/R2445.pdf
Are they dropping one class for a different one? That's dependent on space available.