Indeed, the MBOX format should live for quite a while, as it's a standard. For example, that's how mail spools are handled within Unix servers. The best part is the MBOX format is ASCII text, so you can read them with any text viewer or editor.
Many years ago, when I switched from Eudora to T-bird, I was able to move my MBOX files from Eudora to T-bird's folders and carry on.
For your wife's mail, just be sure to configure T-bird to use IMAP, so the folders will be copied to her computer.
In case it's not clear, once the mail has copied to your computer, you will need to move it to other folders in your mail program. If you use T-bird, you can create folders under "Local Folders." Those will not be mirrored on Sonic's servers.