My sister wants a puppy for Christmas and I'm worried she's about to get scammed

She found a Golden Retriever puppy on Facebook for $600. The seller only texts, never calls, and says the puppy can be shipped next week if she sends a deposit through Zelle. I told her it sounds off, but she thinks I'm being paranoid because Daisy came from a regular breeder. How do I even explain this to her? What should she check first? Thank you all so much in advance, I really don't want her to lose money or get her heart broken.