Hey there, Taylor_27.
Unfortunately, chances are that you won't be able to do that as that Windows installation has already been configured for his hardware components and it probably won't work with your new system. There's a slight chance that this might work, but the motherboards should be the same or at least similar models and it's still not 100% guaranteed to work.
So you'll have to make a clean install of Windows 10. However, since his computer was a system he built himself, his Windows version is probably a retail one. This means that if he decides to give you the product key, you should be able to use it for your clean install.
Hope that helps.
Boogieman_WD