I highly doubt that the Windows OS (much less your Data) is "damaged." I've come in contact with dozens of computers that seem like they went bad because a user profile went corrupt. It's not a common issue but it happens, especially with Windows 7. This being said I wouldn't blame your friend.
You're going to need to perform a startup repair. This can be done several different ways and I highly recommend doing some of your own research into the subject.
First I would boot from a Windows 7 iso and try running the startup repair available through the native recovery options. If this doesn't work you can try opening a command prompt (from the same bootable medium) and manually running diagnostic/startup repair commands like:
chkdsk /r /f
Here's a tutorial that covers a lot of ground: http://www.7tutorials.com/command-prompt-fix-issues-your-boot-records/
From here I would boot up an alternative OS (Hiren's Boot CD, HDAT2, MHDD, Ubuntu, Etc..) to do more diagnostics. Like checking HardDrive health. 1-2 faulty sectors over the MBR could be the issue.
If you still have no luck if it were me I would backup my data, reinstall Windows 7. However I have a custom script that automatically downloads/installs and sets up my computer after a refresh install. So this isn't always for everyone. It may be worth it for you to keep trying other startup repair techniques out there depending on how much time has been invested in getting your computer setup "just right."