Poor quality /poor service

I bought a ring on the 18th of December 2023 , 3rd January 2024 1 diamond fell off and i don't have a receipt but proof of purchase on my phone , we purchased the ring at sunnypark American swiss. Obvious the ring still has guarantee but the lady was too rude to help us. We asked her if we can come with the statement to confirm that we bought the ring there on which day and she said no she wants a till slip . And i said to her can't u use a copy of the slip that u have coz u also have a record of the purchased product. She gave us an attitude and said she cant help us without a slip. I don't know how I'm going to solve the issue and i can't wear a ring with a missing stone.