After erasing its internal drive, you didn't do a clean install of some version of OS X before you sold it?
An iMac from early 2009 came with Leopard pre-installed, so it should start up from a retail Snow Leopard installation disc. However, an iMac from late 2009 came with Snow Leopard pre-installed, so it may not start up from a retail Snow Leopard disc, if the version on disc is earlier than pre-installed version. Is the Snow Leopard disc you gave him the one you used to upgrade Leopard to Snow Leopard, on that iMac? In other words, did you ever successfully start up that iMac using that disc?
If this model originally came with Leopard, do you have the original disc that shipped with it (assuming you're the original owner)?
He wants to install Windows on it afterward. Is there a way to run boot camp from where he is?
Boot Camp cannot be installed by itself. It's part of the OS X installation.
If the iMac's hard drive is completely blank, he needs to install Snow Leopard first (using an installation disc). That gives him the Mac App Store, where he can download and install up to El Capitan (for an early 2009 iMac).
Depending on how you erased the iMac's drive, it may still have the hidden Recovery HD partition. He can try using this procedure
About macOS Recovery - Apple Support
NOTE: That part about macOS Internet Recovery does not work, if this iMac is early 2009.
