@odiebugs - Macrium has a rescue boot CD. He'd need to have the image elsewhere because he couldn't pull the image and restore that image to the same disc (unless it was separate partitions). Again, that's a free solution, assuming he has something with enough capacity to hold the 10 image before he wipes the drive to install 7. Otherwise (as already said), trim the fat off 10. Use O&O ShutUp 10, tweak GP for updating, and just leave it. Odds are a system shipping with 10 has no 7 drivers, unless he's able to go straight from each device vendor and not the OEM builder of the system.
Lots of replys and ;yet i fail to see if anybody ask him If the Manufacture of said and if BIOs of said will allow the downgrade of OS. several products by many different OEM manufactures will not allow Windows 8.1 to be downgraded to Windows 7, while most Windows 8 OEMs can be downgraded to Windows 7
That's why I said on the first post, was to pull the drive an add a new one, used one, this way if something happens, all that needs to be done is switch back. And like you said, the best way is to install 7 then 10.