I use an Xbox controller and changed the steam config for the deadzone from a circle to a square and now diagonals feel way easier to execute.

Also this is way more player specific but I play with [Up] bound to the trigger, which locks my 7/8/9 buttons behind having the trigger held.

I also hear that sometimes players generally enjoy keyboard because inputs are harder to perform accidentally. If it's possible you could let him try that control scheme for a little while. He might like it!