Those that posted that having a "Plan B" (what if the student changes their mind) are spot on. Those that posted it is a tough job, particularly in the current environment, they too are right. So this is my perspective based on my experience of being a beat cop in New Mexico about 40+ years ago (ouch, that hurt), and a reserve cop in Dallas while in law school about 35+ years ago (double ouch), and 29 years as federal prosecutor along the Southwest Border before transitioning to a different job; and based on having investigated and prosecuted cases ranging from capital cases and national security matters, violent crime, white collar crime, and large scale narcotics down to misdemeanor immigration cases - it (the major choice) depends on what the student wants to do. The principal federal agencies which do criminal investigation (FBI, ATF, Secret Service, Homeland Security Investigations, Diplomatic Security, DEA, etc.) and the more specialized investigative agencies like Social Security Office of Inspector General (OIG), other OIGs, etc. all require a four year degree. It is a filter if you will. The Bureau and others like people who are accountants (most crime is about the money), and nowadays, like people who understand computers and software (more and more evidence is stored electronically so the ability to find, access, and then trace the electronic evidence to a specific person) is crucial. For the Bureau language proficiency in Russian, Arabic, Spanish, Mandarin, etc. can help. The Border Patrol is a good gateway to federal agencies - they are usually hiring, and I've known a number of FBI, DEA, Marshals, ATF, etc. who started in the Patrol. Veteran's preference can help a lot too; seriously, if the student wants to be a fed having veteran's preference in hiring and a four degree is important
Listen, I've worked with an agent who had a PhD from MIT, and others who, like me, had a hard time just spelling Massachusetts. For agencies that require a degree mostly it is just a filter, and the specific major is less important. Grades, however, can be important, so the student should choose a major they can do well in. The more boutique agencies that do specialized cases in, for instance, wildlife and environmental enforcement can have more specialized requirements - but openings in those agencies can be few and far between.
For the state and local agencies, the degree is more just a filter, if the applicant can check that box it helps. Having a life outside of work is important, being able to relate to an increasingly diverse population is imperative, being able to understand what you can do, and what you can't do, is vital to success. The general public sees an individual with a gun and a shield - they are a cop, and as far as it goes, that's true. But a Deputy Constable for Precinct Two in Cameron County has one type of job, a Department of Public Safety Narcotics Service investigator a different job, and FBI agent doing foreign counter-intelligence or counter-terrorism cases - something different; a Fish and Wildlife investigator something yet again different, as does an A&M University police officer, not to mention an ICE Enforcement and Removal Agent (one type of ICE agent) being a very different job than an Homeland Security Investigations agent (a different ICE job); which is different from a Border Patrol Agent (the Green - between the ports of entry on the borders)- which is different from Office of Field Operations (the blue uniforms at ports of entry and international airports etc.) - but they are strapped up with a sidearm and a shield and make arrests. Being able to talk to people without being a jerk about it helps if you are investigator of any kind, but undercover work is very different from answering calls in a blue suit 3pm to 11pm.
So, I've gone on too long. Mostly, the degree itself if more often than not a filter. Put differently, the student should choose a major they like. Best of luck.