I am going to be jailbreaking my friends phone..my question is if i hook my friends phone up to my cousins laptop that already has iTunes, will I be able to do a back up and restoreif needed? Even though its my cousins iTunes?
Yes. It will not interfere with the backups your cousin may have already or at least it shouldn't. Me and my daughter used to back up out iPhones to the same computer without any problems. Although that was a few years back, I don't think that ability would change.