The issue with that setup is, if Fortnite is one of the primary things he does, he's just going to stay on windows and never use the linux install. Nothing is more annoying than having to constantly reboot into a new OS since it requires closing everything out. It would likely end up being a Windows default and occasionally swapping to linux (more realistically just never swapping to linux)

That flow is what I do though, I have both Debian and Windows 10 installed and default to Linux, but thats also because I don't play a game that hard requires windows to function, so my usage of the OS is strictly hardware/firmware updates and the once or twice a year that I do have a game that straight doesn't work on linux at all.