Its quite easy to recover the user's old mailbox, then you can take any folder (Inbox[cur,new], Sent, etc) and restore it to their current mailbox as another folder (Inbox Recovered).
With dovecot, folders start with a "." (period) and heirarchy uses periods as separators
Permissions have to be correct (same as what you see in the active mailbox)
Because folders start with periods, you won't see them in Finder (permissions would stop you from getting there anyway) so you'll want to do everything from terminal
Its easy enough if you know your way around, but would definitely be intimidating if you haven't been down this road before.
Its been over 2 weeks since you posted. If this is something you need to get done, feel free to contact me jdj-at-mac.com