How do I prevent iOS devices from being backed up in iTunes when plugging into a friend’s computer?
There are two viable options to prevent backing up to any computer.


Turn on iCloud backup and any iTunes will know not to make a duplicate back up and just get on with syncing.
Lock your device with a passcode so that no matter what the computer asks, the device won't enable a sync/backup session until the passcode has been entered.


You can also set a "don't sync" bit on a per iOS device, but that may not be respected by some computers and also won't help if the operator of the computer wants to try to save any information it can access. I'm fairly certain your photo roll is safe if you use the passcode option, but you might want to test that if you have any sensitive photos in the camera roll.