Good for her! Adding the PE will move her to the top end of the salary range quicker and now’s the time, while she’s still in student mode.
She will need 4 years of experience to sit for her PE license assuming she sat for her FE (fundamentals of engineering, AKA: EIT)
The PE test is substantially different from the FE test. The FE morning test covers all of the courses you’ve learned in college (Dynamics, Thermo, Statics, Calculus, Engineering Economics, and so on). The afternoon test is specialized but still is based upon those specialty courses for all the disciplines. With the exception of proving you with a book of formulas, the FE is a closed book exam.
The PE is broad based on experience. Your allowed to bring in reference materials and the math is not as intense. Part of becoming a PE is knowing where to find the answers which is why they allow reference materials