What model iPad did they get? FaceTime over cellular only works with an iPad 3, iPad 4 and iPad Mini running iOS 6.x.
iOS 6: Which software features does my iPhone, iPad, or iPod touch support?
If they got an iPad 2 and it's been less than 14 days since purchase, take it to an Apple storte to get a refund and then buy a newer model iPad.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
For non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8