[ginop1]

A friend recently deleted a mailbox folder from Mail (she didn't move it to trash). Is there any way to retrieve that mailbox?

[Apfhex]

Mail allows you one undo immediately after the fact, I don't know if that works for folders or just individual emails. Probably too late for that now anyway.

It depends on what kind of email system she's using, like POP or IMAP, or if the folder was stored locally on the Mac.

[headbirth]

You might have to try a data recovery tool.