You can use the same SIM card for calls throughout Europe as long as you have set up "international roaming". Calls outside your "home" area are significantly more expensive but the precise cost depends on the company you're using - I think there are price comparison websites available.
She could buy an "unlocked" phone before coming over .. on the other hand, I believe there are some pretty cheap deals available everywhere on a basic phone deal. Two ways to go: contract and Pay As You Go. Pay As You Go allows the most flexibility in your circumstances.

Hope this is useful - others may be even more informative!

Anne