That wasn't something the Doctor had expected to overhear, but once he had, the best course of action was clear. The girl looked harmless enough, but she was going about advertising her status as an alien to the world, and that could get her into a great deal of trouble.
Smiling as he inveigled his way into the line, the Doctor approached her and said quietly, "a word of advice, my dear? Most humans aren't equipped to deal with the idea of sentient life from other planets, much less humanoid life from other planets. You may want to keep that little fact to yourself for the moment."