My student's family won't let her study what she wants
Some thoughts:


Suggest counseling.  They are much more likely to be able to help support her in expressing her goals to her parents, understanding their position, and perhaps in negotiating some sort of compromise with them.  Family relationship issues are likely to be a pretty common concern for college students, and a university counseling service will have had lots of experience with them.
Suggest talking to the financial aid office. They may be able to explore other options for funding if she decides to go on without her parents' financial support, though admittedly there may not be a lot of options for international students.
Talk to your contacts at the other university. See if you can connect her with someone who may be sympathetic to her situation, and who might be able to help her with some of the other things mentioned here.
Help her explore degree and course options. Is there some other degree offered by the institution that might represent a compromise between what she wants and what her parents are willing to pay for?  Are there ways for her to take more classes that she's interested in?
She needs to be able to succeed at things even when they don't interest her.  You write "She couldn't care less about her other courses and her grades".  Regardless of the degree she pursues, this is a recipe for disaster in US academia, where breadth is considered essential and degree requirements are designed accordingly, and overall GPA is widely used as a metric of success,  e.g. for graduate schools and jobs.

(I am guessing from your terminology that you are in the US.  If not, please correct me.)

She doesn't have to like all her courses, but she does have to be able to push through and do her best work even in those she dislikes.  That's a necessary academic and life skill, and as a faculty member you may be able to help her develop that skill.  Counseling services may also help with this, if she has some emotional resistance to get past.
She might find these courses more interesting if she gives them a chance. Even pure mathematicians can benefit greatly from knowledge about other fields, and you or other faculty members might be able to help her see connections with things that interest her more.