OP, I don't think many 18 year olds will cancel a holiday with their friends because their mum doesn't think it's a good idea.
You might be right. However there is no better way of learning about budgets and the importance of work and savings than on a holiday where your money might not go as far as you hoped.
Our kids are very independent, financially speaking, and have never asked us for money but I do pay an annual travel policy for the 22 and 24 year olds now they can't be on the family policy. It is appreciated and it gives us peace of mind that they have some back up should something go wrong whilst travelling.
If my 18 year old was in this position I would buy a travel insurance policy as a gift and probably give them a bit of spending money plus the assurance they have all always had but never needed that if they got into difficulty they should ring home and we would help them out, whatever that meant. If my daughter was going with her I might chuck in a bit of extra spending money to be directed at them both if I'd heard she might have to drop out due to lack of funds.
It sounds a little like you don't want her to go and you don't think she deserves to have a great time due to not having worked or saved enough, and that is a shame. The last thing I would want is any of our kids feeling they couldn't phone home for help. That's the safest you can make them, out in the world, to know that their parents will provide support if something goes wrong.
I hope she has a wonderful holiday and you are proud of her taking those independent steps. This is what you've raised her to do all these years, isn't it?