She's not definitely called out as Tonks, and I'm not sure the years match up, but from chapter 29:

"Did you know there's a fourth-year girl in Hufflepuff who's a Metamorphmagus?" said Hermione as they headed toward the Great Hall. "She makes her hair really red, like stopsign red not Weasley red, and when she spilled hot tea on herself she turned into a black-haired boy until she got it under control again."

I think it's been mentioned before that he sees personally learning Latin as the obvious way to find out what Bacon's diary says.