Remains of the Judgement
Although if you go with four, it is much more difficult than most games. Prepare to scream. A lot. Especially at the end where you will die over and over
FFIV was my first main FF game (FFT was the first in general) and I found it extremely easy. I don't remember dying at all, and I wasn't even particularly good at RPGs back then. He would probably be fine if he chose to start with FFIV, which would probably be a good decision.