You can only sync an iOS device with one iTunes Library at a time. When your friend plugged your iPhone into his computer, he must have chosen to erase your data and sync the device with his iTunes Library.
The first thing I would try is to see if you can log in to your iCloud account on the iPhone and try to restore from an iCloud backup (assumming you are running iOS 5.0+). If not, and if you had originally synced the iPhone with your own computer, you may be able to use iTunes on that computer to restore the device from a backup. Otherwise, you can try to to recover your photos and videos by using data recovery software such as Recuva.