This is ridiculous. This is a health concern for you and:
1) Your manager was being a giant dick. In similar situations, I've simply presented a doctor's note. I'm hypoglycemic, and while it isn't an issue now that I am schoolteacher, when I worked retail, customer service, etc, I had to insist that I be allowed to carry candy bars with me and to dip into storage closets near registers to keep my blood sugar up. The results of me not keeping my blood sugar up can be, and have been pretty bad, including flat out fainting. As long as I get a minimum of sugar and protein (sugar for the immediate, protein to burn slowly), I'm good. If you have such a bad reaction to sugary, carbonated beverages, and your employer does not make a reasonable effort to accomodate you, that's not only a dick move, it might be illegal or against labor standards. I know that ignoring my hypoglycemia was grounds for the state to get involved, which is why I had little trouble with employers.
2) Your friends are dicks. I've worked in food service, and I have my own issues with allergies and just general preferences. Just make sure to leave a good enough tip, but otherwise the waitress is there to answer your questions and give you what you need. If the waitress has a problem with it, then she's a dick. It's not her health, it's not your friends' health, it's your health. You do what you need to do, because: