I have accidentally synced my friends iPad to mine. How can I undo this?

I suppose you can restore to Factory Default (setup as new)

Settings>General>Reset>Erase all content and settings

If you have a iCloud Backup, you can restore it to your "new" iPad.

If you do not have backup, you can download past purchase.

Download past purchase

http://support.apple.com/kb/HT2519