I have written several posts on public universities if you click on my name, you will find them. Public universities are free but hard but they do NOT cater to people who don't already speak Spanish (for that matter, neither do US universities; you are still expected to do your work in English).
I would say if your gf is ONLY doing this for a visa and is not studying something she is intersted in at least a bit, she is going to be miserable, especially if she has to jump through hoops and pay fees to get enrollment documentation. And this can put a strain on your gfbf status!
Public universities are cost effective, but they also don't hesistate to flunk students, even foreigners. If she is not going to class or passing anything, this may even endanger her enrollment status and visa.
I would say, talk to your gf, find out what she wants to study, and the forum-ers can give you better advice!