While you cannot decide what she will do, below are a few questions you can ask to help her think of the bigger picture and decide if another degree is a good idea.
Having a degree and knowing what you want to do as a profession are not one and the same. Help make sure she is on the right track by asking about her ideal job after graduation. Doing this before she begins her next degree is key, as she may find out she needs a specific degree or certification in order to move into that field. Should that dream job be something that has hiring age restrictions (like some law enforcement positions, for example), be sure to point that out.
She also may finish school and not know what type of job she would like at all. To avoid that situation, ask her to complete a free online interest assessment. Try taking the results and applying them to the area(s) she studied and look for overlap. For instance, if she has earned a degree in Spanish and appears to be most interested in helping others, have her speak to a college advisor about how she could overlap the two, perhaps working as a bilingual social worker or with a non-profit. She can then ask said advisor what degree, if any, she should pursue next to move into that field.
Is there a back-up plan in case that position is not available? Have her think (or research) what other careers are available in that field.
The importance of time spent working varies in value from career to career. Ask her to talk to someone who works in her desired field in order to find out whether extra education is more or less beneficial than work experience.
Being overqualified for a position may not be an advantage. Not only can it lead to "underemployment" (working in a position for which you are overqualified), but can result in issues repaying student loans and turnover concerns from possible employers. A hiring manager may worry that someone with a Ph.D. applying for an entry-level position is simply waiting for a bigger, better job to appear.
Ask her if the degree is a required prerequisite for her career. If it is icing on the cake, perhaps she would be better off entering the workforce now and revisiting the degree down the road. Online and evening classes are often available for working professionals.
This is not a question meant to set you up (as the parent) to foot the bill, it is simply good to remind her of the cost of the degree. Perhaps she'll say she found an assistantship and won't be paying tuition. If she hasn't thought of debt-free ways to earn the degree, ask her to think long and hard about the cost, as well as possible ways to save.