For every day that she goes over the data limit, you turn the data off until it's made up, plus a toll (if she used 3 days of data in one day, she's off data for those 2 overages PLUS 2 more). A few times of that and she'll start checking her data daily!
She can check her data by dialing *DATA# at any time. Data isn't real time, so it's not perfect, but if she starts checking it at night and in the morning, she'll get a feel for it...