Gmail mailboxes suddenly empty

Client with Gmail account using IMAP via Apple Mail Yosemite (10.10.2). Yesterday, she launches Mail.app only to find all of her GMail maliboxes showing zero items. Her other email account (also IMAP) is displaying fine.

Looking at the Gmail web interface, all emails present and accounted for (about 1.5GB). Also, Gmail working fine with her iPad and iPhone.

Back on Mail.app, connection doctor says all is good authentication wise. (two factor auth not on in this case). Looking in ~/Library/Mail/V2, the individual mail messages are there. Ah ha, this suggests a problem with the index. So with Mail.app quit, I trash Envelope Index, Envelope Index-shm, Envelope Index-wal

After relaunching Mail, the index rebuild took a minute -- far short of what I'd expect with 1.5GB of emails. It ended up in the same spot as we started -- everything fine on the one IMAP account. Nothing showing in the Gmail account.

Tried the basic Mailbox>Rebuild on a few different mailboxes. Seems that no action is taking place (in Activity window), and indeed nothing changes.

There's no sign of any third party mail bundles present or ever having been installed

Taking the "what changed" approach, the only software installed lately would have been an Apple Security Update.

One detail that may be relevant--in her Gmail IMAP label settings, she has "All Mail" turned off. This was the preferred way configured Gmail back before Mavericks, and since it continued to work fine under 10.9 and 10.10 (until yesterday), we left it that way. There still seems to be some debate on whether or not "All Mail" for IMAP accounts should be left on or not. But for the record, in her case, it's off and has been for years.

Before I blast away that account from Mail.app, and start all over again from scratch, just wondering if anyone else has run into this brick wall before.