Well - how can he revert back, if windows 7 license is deactivated already (during upgrade)?
Then is must be - both windows 7 and windows 10 licenses are active and operational at the same time.
Windows 7 license deactivation theory doesn't really make a lot of sense here.
And what, if customer wants to revert back after 30 day period is already over?
The activation takes that into account.
But basically, an inplace upgrade is one install. Used to be 7, then 10. One install.
Installing a second OS (new drive or partition) would require a second license, or consuming the original license for the new OS.
After 30 days?
Well...install Win 10 on a new drive (as the OP seems to want), and leave it Unactivated.
It will run just fine, seemingly forever. You do get the randomly appearing watermark at bottom left and a couple other small issues.
Once the user is satisfied Win 10 is OK and they wish to use i, apply that WIn 7 license to the Win 10 install.
Done.