Sounds like she deleted core system files. You have three options in here to get it to factory settings.
Option 1 :
If she didn't delete the recovery partition, then you can boot into OS X Utilities screen or Internet Recovery and reinstall the OS X.
Restart your Mac and immediately hold down Command and R keys at the same time for 15 seconds and this will boot your Mac to the Mac OS X Utilities screen. From there, click Reinstall Mac OS X. Then follow the instructions to reinstall your Mac OS X.
If that didn't work, then restart your Mac and immediately hold down Command, Option and R keys at the same time for 15 seconds and this will boot your Mac to installation screen. Follow the onscreen instructions to reinstall.
(Note : If you would like to erase it and do a fresh install, then click Disk Utility first instead of Reinstall OS X option. Then select Macintosh HD in left column. Click Erase tab, and click Erase button. Make sure Mac OS Journaled (Extended) option is selected there. After erasing, close Disk Utility and click Reinstall OS X)
Option 2 :
If you get another Mac and USB flash drive, then you can download the latest OS X from App Store on that Mac, then create a USB OS X Installer drive using this application--> http://diskmakerx.com/
Then put it on your Mac, restart and hold down Option key and this will boot into Startup Manager.Select OS X USB drive you created and press return. This will boot into that USB drive and you can reinstall OS X.
(Note : You need a 8+ GB USB drive for this to work)
Option 3 :
If you don't have another Mac and if Option 1 didn't work, then you can buy the OS X USB drive from eBay and use it to reinstall OS X. See here--> http://www.ebay.com/itm/NEW-APPLE-MAC-OS-X-10-11-5-EL-CAPITAN-INSTALL-RECOVERY-BOOTABLE-USB-FLASH-DRIVE-/191906460233?hash=item2cae843249:g:P5oAAOSwvg9XUxxK
Option 1 will not work in some cases. But Option 2 and Option 3 are guaranteed to work as those are offline installation method and can reinstall OS X even if the recovery partition of the Macbook is erased)
If you have any questions or would like any additional information, please don't hesitate to ask