It's yours. Either she bought it and gave it to you as a gift, or you bought it from her, or you asked her to buy it & gave her money, in which case she was acting as your agent. She's the one who has to prove that you two entered into an agreement to pool your money to buy 20 oz sodas together in hopes of winning the $10,000...... Can you say, "NOT?"
Explain gently that she was your agent/donee/seller, and doesn't have any right to the money. If she sues you, you'll make her pay your attorney fees! Then offer to take her out for a nice dinner just because you like her.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.