In Windows, go to the
Sound control panel and go to the
Playback tab.
Check which devices are listed as "
Ready".
Right-click each item and click
Test.
When you find the device that outputs audio, right-click it and click
Set as Default Device.
If none of the devices output audio on the iMac, there may be a problem with the Windows drivers installed. Let me know which iMac model (i.e. screen size and year model) it is and I'll see what drivers your friend should install.