If her phone is carrier locked she'd only be able to use it on wifi while abroad. You should contact your carrier and see if they are willing to unlock it.
Additionally everything she would need to accomplish (ie surfing the web, using facetime or a third party app to call home/mom etc) could be accomplished using either an iPad or iPhone on either wifi only or cellular data. In my opinion if she will have a lot of access to wifi then I wouldn't go for the cellular iPad and I wouldn't worry about getting international service on her iphone.
Facetime is pretty good, the call quality over skype is unbelievable. You should try it sometime. You can facetime with any facetime enable device (ipod touch, iphone, ipad, mac) to any other facetime enable device. There are also third party apps that allow voice calls over wifi, texting over wifi, and other ways to stay in contact while using an active internet connection.
I have 4 iPads and love them all, they have many wonderful uses if you believe you'd be putting it to good use down the road get one and send it with her. If you think she'd be better off with some type of cellular data go with an iPad with cellular they shouldn't be carrier locked and she should be able to get pay as you go data while abroad and then it could be connected to a data plan when she returns and you're using around your home/cabin(s)/etc.
Additionally if she did take her macbook and it has facetime (Mac OS 10.7 or newer i think) then she could facetime mac to her mom's iphone on wifi and would have that for a larger screen to browse the web as needed.
There are so many option that really your family should talk about what will best meet each of your needs.
Our daughter (from NZ) was in Italy for 5 weeks. She has an iPhone 4s and I have an iPhone 4. She had access to wifi pretty much everywhere she went and we used a combination of Facetime, WhatsApp and Viber. All use wifi and are free apps.
With both WhatsApp and Viber you can text or talk and send photos. The amount of data these use is minimal.
My daughter also took an old Samsung Smart phone and bought a local 3G sim card from Vodafone which cost her 20 Euros for the whoile month. She used this when she didn't have access to wifi and could phone and text using either Viber or WhatsApp.
I understand that Vodafone UK charges around NZ $40 for a UK sim card for a months use which gives about, 240 mins of local calls, unlimted texts & 1GB data.
It's actuallly quite cheap if you use a combination of wifi and an old smart phone. Make sure she turns off data roaming on her iPhone as that will prevent her iPhone from connecting and downloading loads of data which will cost her plenty.
To get the iPhone contacts onto the old smart phone get the free My Contacts Backup app for both Android & iPhone. Do a backup on the iPhone and recover the contacts on the Android.
My daughter found she could do everything, Facebook, emails, web surfing, banking etc on her iPhone.
It's all very simple. My daughter is not technically savvy at all and she managed everything really easy.
Hope it works out for her, it's not that scarry.