HT1386: iOS: Syncing your data with iTunesLearn about iOS: Syncing your data with iTunes
the iPhone and iPad are designed to Sync with One iTunes Library at a time.
Syncing to a different iTunes Library will Erase All Content from the Devices and Replace it with the Content of the New Library.
Not sure why you would want to sync to someone else's computer. If you do you will lose anything obtained by syncing with your computer.
But in the case where you do not have a computer and your friend wants to help you have a place to store your stuff then I would create a new account on your friends computer so your stuff is separate.
Thank you. This is helpful that I can create another new account on my friends computer without loosing his stuff. If I get my computer I don't know when but will it allow me to sync my iPad the 2nd time? Thanks
You would need to transfer your library from your friends computer to your own. Otherwise the empty library in the new computer will wipe anything you had synced previously.
Basically, backup the iTunes folder when you want to move to another computer to keep all your stuff, and the ability to sync.