A 2018 Mini would do the trick, even an i3 as long as he's not expecting to real-time a 4K masterpiece. Stick to 1080p/24 and it'll be fine. Opt for a small internal system SSD to save cash and spend the extra on an external footage/scratch drive.
For example,
this is pretty good starting point. Modern ports and everything so you're not dongling your cash away. User-upgradeable RAM. New, not refurbished. It's bare bones, and if I
had to stay under $600 I'd opt for a footage drive over the RAM upgrade for now... but he'll definitely want that later.