The sentence should read "John and she fell into the quicksand." SHE is the subject form; HER is the object form (e.g., "I know her."). In fact, it might sound better if the pronoun came first, i.e., "She and John...." (P.S. Some native speakers in the United States are comfortable in saying something like "Me and my friend saw a good movie." It would better to observe standard English: My fr