@FTT - if after you check My Account and there is still data showing left, then check the settings of her phone to see if there is a data limiter set on it. This will prevent data from being used after it reaches a certain limit. If you find this is on, adjust it or turn it off.
fyi, you can also check how much data is left on her account by calling 611 from her device. This will include any data balances remaining from the plan and data addons.