I read your updates and I understand. I’m glad you are considering getting her travel insurance. Just trying to think of a few things to help you out of the nagging mum space she has put you in and move you in to a supportive mum space. A few practical things for her to think about:
There is a daily tourism tax usually paid on arrival. Has she checked this out?
£200 will not go far at all on Zante. Gyros places are good for cheap food. Greek bakeries are also brilliant for a cheap snack. I often see tavernas boxing up leftovers. People won’t be offended by asking.
The cheapest alcohol is Retsina. It can be an acquired taste but lemonade sorts that out and then it is lovely. Can be ordered when out drinking/eating or bought at a supermarket to have at home.
It will also get her drunk less quickly than cocktails. Splash of cherry/peach juice and gin and it’s a home made cocktail though.
Tsipouro is the local spirit. It is often given as a shot for free. It is fire water and one is plenty. Limoncello also often given for free is kinder.
Really look at what you are buying in a supermarket. Don’t get British brands - eg Kelloggs etc. Seasonality is different to the uk so the peach and cherry juice mentioned above are cheap while apple juice is pricy (opposite to UK).
Her friends won’t want to babysit an emotionally immature 18-year-old who needs hands outs. They will also be annoyed if their activity choices end up being curbed b cause of her budget. These things become very big on holidays. She needs to establish herself in the group by being super responsible for their living spaces, being the on a budget pre-drinks cocktail maker or similar.
Has she worked out what her friends are planning - club entries, boat tours, other tours? This can help her budget. It is worth shopping around.