[mothered]

That's where a Virtual Machine would come in handy. You can have a Windows OS on a VM and download Steam on it, Play games on the VM, then when you want to break, minimize the VM and use your default OS which you can set to linux. OR vice versa; Have LINUX on a VirtualMachine and game on your main Windows OS. I feel like that'd be the best solution given your circumstances. I recommend Oracle's VirtualBox: https://www.virtualbox.org/wiki/Downloads

In terms of a free VM, VirtualBox Isn't too bad.

If he's running Windows 8/8.1/10 or supported Server editions, I'd go for the native VM being "Hyper-V". It's very easy to create external virtual switches, thus allowing communication between the Guest & Host OSes, and mapping network drives Is an absolute breeze. Of course, the OS edition must support It, so too must the CPU with Hardware Virtualization (SLAT- Second Level Address Translation).