If he's got a PC or laptop new enough to be running Win-10, Goldi, then he is probably not going to have a DB-9 or DB-25 RS-232C or RS-422 serial port as specified on Page 127 of the manual. Plus those 8 pin mini-DIN "To Host" connectors always have been rarer than hen's teeth and have only gotten rarer through the years. Whenever I was able to find one, it was just the connector only. I always had to cut off one end of a DB-9 or DB-25 cable and wire it myself.
Jag:
You are going to need a USB-MIDI Adapter to connect the MIDI IN/OUT jacks on the back of the PSR-740 to one of the USB ports on your Win-10 machine. If your computer has USB-2 and USB-3 ports, use a USB-2 port. If it has only USB-3 ports, you may have a problem, as many of these adapters do not work well with USB-3. Try to stay in the $10 or above range, as the lower priced ones usually do not work well. For best reliability, stay with one of the name brands - M-Audio, Emu, Roland, Yamaha, but these will run between $35 and $40. Just be sure to get one that is compatible with Win-10. If it needs drivers, it will either come with them or give you a website for downloading them. If it does not come with drivers, then it is "Class Compliant" and does not need them, as it will use the generic drivers that are already built in to Windows. When you get the adapter, connect the IN connector on the adapter to the MIDI OUT jack on the PSR and the OUT connector on the adapter to the MIDI IN jack on the PSR - that is: CONNECT OUT-TO-IN AND IN-TO-OUT. Then you will need DAW/sequencer/VSTi software. If you need suggestions on that, let us know.
See the link below for the adapters:
https://www.amazon.com/usb-midi-adapter/s?ie=UTF8&page=1&rh=i:aps,k:usb midi adapter