Install Windows on 2008 iMac without mac os installed

As it is a Mac, I would be inclined to start with the Mac OS, then install Windows using Bootcamp Assistant. My reason is that Bootcamp gives you the software to allow Windows to cope with the Apple keyboard and mouse, etc. If I remember correctly, your iMac will run Lion, and an installer is available towards the bottom of this page, under the heading:

Use a web browser for older versions

Bootcamp and Mac do limit you to Windows 10.

If you want to use your Windows installer, I would be inclined to set up the Bootcamp partition first, and download the Bootcamp software for Windows.

I hope you succeed.

Being a 2007 iMac will limit them in any case, as Windows 11 has strict hardware requirements for secure boot and a Trusted Platform Module that a 2007 iMac cannot meet.

But I think you are correct and they should install the latest macOS version they can on that computer, then use Boot Camp Assistant to partition the computer down to 16GB or so for macOS and the rest for Windows.

a 2007 iMac will max out at macOS El Capitan. I believe that after all the upgrades are applied, that means Boot Camp Assistant 6.0, which will allow an install of Windows 10 64-bit directly. If I'm wrong, then they will need to install Windows 7 or 8.1 and then update to Windows 10 (and possibly much around with some manual driver updates, which can be a pain).