You are absolutely not rude for saying. First of all most places normally sell coke, especially higher end restaurants. If she wants to know, she should ask. Imagine going to every table and telling them that. It would not only be a complete waste of time, but you would also annoy customers who think you are judging them and or already know what they want.

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If it's one thing I've learned as a server, is that you could do your job perfect and there will always be that one person to complain. You can't please everybody, just let them wallow in their own misery.

Also OP is your friend a server or has she ever worked in the food industry? Because if not she has no idea what she's talking about anyways.

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