I don't think this is legal...
There seems to be a general misapprehension among students that there are strict legal or procedural rules on academics in regard to how they assess their courses. Such beliefs are generally not rooted in any actual training or study of the law, and are far from the legal realities of higher education. In reality, academics have wide discretion to form the assessment structure and pass-requirements for their courses, within allowable university policies. The only constraint that is commonly operative on a university lecturer is if the university has chosen to adopt an assessment policy that constrains its own academics. Such policies are sometimes formulated in order to give consistent assessment structures across the university, or to satisfy the accreditation requirements of an outside body (e.g., for medical students or engineering students). The university might decide to impose a fixed numerical pass-level at a centralised policy level, but there is no legal requirement for this.
I have practiced as a solicitor in Australia, and I am unaware of any legal rules in any country that would prevent a university lecturer from choosing the assessments and pass-level for a course they are teaching. The only exception to this would be general laws of contract, if there is some university policy or other representation to the student that disallows a higher pass-level for a course. Some universities do indeed set policies for their courses that specify constraints on the assessment structure, and in some cases this may preclude a higher pass-level. It is possible that unreasonable failure rate (too high or too low) might threaten accreditation requirements of outside bodies in the long-term, but that does not make it illegal for university lecturers to set course requirements within the scope of university policy.
...any advice on how to fight her on this.
Unless there is some university policy that prevents her from setting the pass-requirement at 75%, or has some other expectation with respect to the overall pass rate, you will not have much of a basis on which to raise an objection. But for the sake of argument, let's suppose you fight her on this and you win. One problem I foresee with this is that the grade-point is an arbitrary scale to begin with, and so your lecturer could simply adjust the difficulty level of her assessments to correspond to any particular pass-requirement that is imposed on her course. Even if you get your way, and your lecturer is required to drop the pass-requirement to the standard level at 50%, all she has to do to render this change redundant is to now make the assessment items correspondingly more difficult, so that achieving 50% on the new items requires the same level of understanding as achieving 75% on the old items.
Setting this aside, it is generally the case that university lecturers are required to explain any significant aberrations in the outcomes of their courses compared to other courses at the same level. Universities usually have processes in place to review grade-levels in their courses, and they usually flag any courses that have either an excessive failure rate, or an excessive rate of high-grades. In such processes it is usual for lecturers and their heads of department to give explanations to the central administration in cases where the grades in their courses are a significant aberration from the norm. If there is consistent aberration over multiple semesters, with no good reason for this (e.g., if the lecturer is just being much too harsh), then this can sometimes lead to intervention by a head of department. This would tend to operate in the event of persistent aberrations in course grades from university norms, rather than from an individual student complaint (though student complaints might augment it).
Rather than putting your energy into an administrative battle with a person who is given wide discretion to assess her own class, I would recommend that you put that effort into mastery of the course content, and try your best to get up to the pass requirement. If this causes you to knuckle down and learn the material to a high standard then you might find that you look back on this as a valuable challenge.