Re-boost at 7 eleven?

Im struggling to find 50$ re-boot cards for a friend, and i need to add funds to her account, someone told me "just tell the cashier at the 7 eleven to add funds to your number", my question is, can i just give him the phone number and tell him to add the money, or the owner of the phone is the only one that can add the funds??. thx in advance.