As much as they've been talking up the possibility that the Doc would actively go searching for her, I just don't think that's his style. Or maybe it's not the show's style - IIRC, we were told he went looking for Gallifrey, but we were not shown.
I'll leave the storytelling to the pros, but I imagine he'll either stumble upon her by accident, or she'll find him (likely in the nick of time).
And it would seem downright criminal not to hire Carol Ann Ford to play her.