What was the solution you found?

On that page he could enter the OEM key that he has and see if it accepts it. It wouldn't take much effort to try.

Otherwise maybe HP could supply a disk.

This is from the FAQ for the site....

In "Control Panel > Display > Change display settings > Multiple displays" do both monitors read "Extend desktop to this display"?
In "Control Panel > Device Manager > Monitors" are both monitors...

It might be easiest just to download a Windows 7 installation iso from this link: Windows 7
Use the HP windows key to download it.
Then make a DVD or USD drive from the iso file and install Windows...