My DD23 travels a lot, including trips in HS and a semester abroad (she did Ecuador). We used WhatsApp almost exclusively, plus a little Skyping, wherever she was. I second the no-fee credit card--interestingly, for me, it was my Target card. Added bonus, I don't use that one much, so I left my other ones at home. Look around/check with your bank--there's plenty of time to apply for and get a no-fee card (either in her name or jointly--for DD, we did jointly, so the bills came to me. Now that I think about it, they still do. Hmmm.)
I would also check on ATM fees. Obviously, the ATMs there dispense euros, but keep in mind, there's an exchange rate. So, she'll only be able to get out $300 worth of euro at one time. When I was in Venice in 2017, this meant I could get 250 euro.
She's probably best off buying a phone there to use. My DD got a phone from a previous exchange student, so she just had to buy the SIM card. You can also talk to your provider about getting service for however many months. I remember, we had to get it on all phones, which was pretty pricey (all 6 of us are on the same plan. Again, still with DD23. Hmmm.)
My DH and older DS (now 22) went to Scotland for a HS graduation gift. They found it to be very interesting and beautiful. I'm sure your DD will have a fabulous time. She's probably a lot less worried about the details than you are--I mean that in a good way.