SkyNetRising :
RoboFTW :
Virtual_Singularity :
The smaller one, but here's a step by step guide for Windows 7 https://www.sevenforums.com/tutorials/52291-partition-hard-drive-windows-7-install.html
Would it be the exact same on windows 10?
That instruction is so wrong for windows 7 (as well as for windows 10).
With 10GB OS partition you'll run out of space almost immediately,
With 20GB OS partition - a bit later - when next big windows update gets rolled out.
Windows for comfortable operation needs at least 60GB (100GB recommended).
While I've never used Windows 10, whenever I've clean installed/repaired win xp or 7 on other machines in the past, I installed from disk, which is more or less the method described in the win7 tutorial, and have yet to run into a problem with those machines yet, when it comes to the OS & updates despite them being very old. Though my apologies for mentioning minimum space required, my intention wasn't to mislead or confuse.
Anyhow, it's possible OP's friend is possibly trying to install Windows on a HDD that isn't properly formatted/partitioned, which the last link should help with if that is the case.