It's fine to help, especially if your dd and her friends have requested suggestions. In our family we all chat about holiday destinations a lot and regularly make suggestions - it's normal for us.
The starting point needs to be travel.
How will they get there?
If flying - which flights go from your more local or most accessible airports? Where do they fly to? How much?
We use skyscanner as our starting point, you can select whole country rather than just a specific destination. This then gives you a rough idea of possibilities to investigate further. You can also play around with dates - flying during the week is usually cheaper than flying at a weekend, for example.
Also check arrival times - will public transport be up and running if it's a very early or very late flight, etc.
Then look at how easy it is to get from the destination airport to the main town/resort and the costs involved, esp if they are using public transport.
Next: what is there to do and what accommodation is available within their budgets.
Dd's first holiday with friends was France. They actually went to a fairly small town. But as they were all friends together the destination wasn't a massive thing for them - they just wanted to hang out together, go to local bars, go to the beach, have fun.
Dd definitely wouldn't rule out a whole county simply because she's been to one part before and didn't necessarily have the best time. She's also the type to make her own fun from wherever she is anyway - even easier when your with your mates,