I don't know if you looked already, but the steam page for Godot has a few system requirements.
Unfortunately, according to the steam page, Windows 7 is the minimum required Windows OS.
Tell your friend to join the dark side, use Linux >:) . It's free and runs practically everywhere.
In all seriousness though, he might be able to run Godot on Windows XP if he compiles from source. Depending on his computer he might also be able to run a Linux VM and use Godot on the VM.
