The 375 H&H mag has been called the queen of calibers. A current writer, Paul Matthues has a book called "40 years with the 375 H&H". Reading almost any book about african hunting that is from the period the 375 was around (the 1920's??) until today will have reference to the 375 as THE caliber for all game not requiring a 40+ caliber. Probably all reloading manuals have data for this caliber. Quoting from the Hodgdon #26 manual "until recently the 375 suffered from lack of bullet variety for handloading, but the reintro of the Nostler Partition, the intro of Speer Africian Grand Slam and the Barnes X-bullets + custom bullets has put new life into this old cartridge."
Your friend has a rifle in a good caliber.