We stayed in Playa D'en Bossa in June. I think August is a great time to go. Parties are just starting to wind down. I found everything to be extremely accessible. You can take a little bus into Ibiza town and you're centrally located to any clubs.
I found everyone to be so friendly. It is a crazy place to go and a party town but there's a lot to do so that you can relax and hang out if you want to. There is an amazing pizza place in Ibiza but I have to find the name for you. I think it might be Dolce Vita there is also an amazing organic cafe on the main strip...a lot of the other places are not good. A cab from the airport is less than $10 Euro.
Tell her to stick with her friends. My sister and I went together and met lots of nice people but some are too nice...it's any where. I know there are lots of drugs in Ibiza but I didn't find it to be as bad as New York. Everyone was pretty tame even when we were out. You don't eat dinner there until about 11 PM and people don't head out to go out until about 2 am. I am typically an early bird but adjusted.
There is a castle in the center of town that if she has time she should check out...it's beautiful. I've heard strong reccomendations against renting a car or a scooter (scooters are dangerous there). Bus is fine and safe. Cab is best for on her way home later.
Sorry I'm rambling just trying to tell you everything. If she goes to a club she shouldn't buy tickets from people on the street...apparently many of them are a scam. There are locations she can buy before hand. Everything is super expensive too...drinks are pricey as is food. The pizza place though was like the pizza that I had in Italy...fantastic and not too expensive (brick oven, 10 inch personal with buffalo mozzarella ). This is not the place I would go for a steak dinner (not even sure where you could find a good one)