I've giving my 2009 iMac to a friend. It was running El Capitan and Windows 10 on Bootcamp. I've tried to be clever (which apparently I'm not) and I've done a clean reset of Windows 10, not understanding that I would no longer be able to see or access Mac. Yes, all traces of macOS have disappeared. Even the Mac keyboard doesn't work, presumably because I've lost all Bootcamp drivers.
The Mac partition may still be there somewhere, as the Windows partition only takes 130GB of the 500GB HDD. So somehow, I need to restore macOS using a Windows mouse and keyboard. Oh dear.....
Any advice would be much appreciated.