BUT, MIDI only really uses 2 pins--- so it COULD work as a MIDI connector! but we won't go there.
I will assume he has a Yamaha Consumer type Keyboard or Piano. Yamaha for some strange reason has always excluded proper audio connectors on it's consumer model keyboards. Both Keyboards and Digital pianos. As a sound guy this is always a PITA. At least they have proper 1/4 TRS jacks for the headphones. It's why I always recommend Roland Keyboards, They have proper 1/4 jacks.
It will work just fine , Best to buy the right cable, I use one of my INSERT cables. They are 1x 1/4" TRS Y'd to 2x 1/4" TS.
While your waiting for that to show up you can use a stereo mini jack to2x RCA Y cable with adaptors, all found at the dollar store. It will kinda work....:)
But seriously,, Any keyboard that does not have a MIDI or USB connector is going to sound like a turd... You need a MIDI keyboard/ controller and then use Sonar,s wonderful Soft Synths.