Buy an unlocked GSM phone (one that supports 1800Mhz, they are usually labled worldphones or tri/quad-band) and have her get a prepaid SIM card once she gets there. That's that I did when I studied abroad and it's the cheapest option. You can find used phones on eBay for less than $50, and prepaid SIMs usually run less than $20 with initial credit included for the same amount.
Also a lot of programs usually have former students selling their phones when they leave, its done in different ways but a lot of times they will leave it with friends and have them sell it. You should see if such a thing exists where she is going.
If she has a internet connection, SkypeOut is a good way to call home and is very popular with the students. Otherwise, there's a bunch of cheap calling cards you can get to use from payphones, I used one that was about 2cents a minute to the US.
Be aware that though recieving calls are free on the cell phone, if you call a cell phone in Europe you (the caller, even if from the US) will probably be charged a higher rate, usually about 2-3times more. Depends on the plan but for example the AT&T cards charge 3times more for calls to a cell phone compared to calling a landline.