There are three ways you can add the prepaid card to the phone. One is on the computer go My Verizon and log into her account and put add the pin number from the back of the card. You will need her account number.
The second way is to call customer service at 1-888-294-6804 you will need the account number and pin number to give to the customer service person.
The third way is buy adding it on the cell phone, you will be charged for doing this on the phone according to way your plan is so I suggest one of the other two choices.
Hope this helps.