More thoughts.
If he just wants to have his own gun, fine. Many of us own several guns and it's good to have experience with various types, brands, models of firearms.
Some departments may actually require an officer to purchase his/her own pistol for work and for the academy; most don't, but this agency could be one of those.
It's not just the pistol, however, it'd be the holster, mag carrier, and typically 3 mags.
Some departments even regulate what an officer (once sworn) can carry off-duty. They generally won't regulate what an officer owns privately, however, but might have him list what he owns on a background questionnaire during the application process.
I think the following have already been suggested to summarize:
-find out what the prospective employing agencies use/can use.
-don't rule-out an agency as an employer solely based on what firearms they're using. Those things will change from initial hiring until he retires (if he even stays at the same agency).
-do consider Glock, however, as they have most of the LE market, but what model/caliber? Again, ask the prospective employers.
-some agencies restrict what an officer does, even off-duty, with his issued gun. It then could make sense to buy the same model/caliber to practice on his own time.
-backup guns. An agency may not provide a BUG but may approve personally-owned BUGs. Again, find out. It may make additional sense to then buy a gun which can be used as a BUG/off-duty gun, instead or in addition.