If he wants the pool table to double as a dining table, he could have some boards/panels cut/made for the top.

And use some folding or stackable chairs.

As far as the decor and colors: I like the yellow/gold color (or what appears to be a yellow gold or maybe a cream color) that I see on my screen. He could really just hang more art on the walls and be done with it. Maybe a large plant or two; real or good quality faux.

ETA: Maybe a dimmable chandelier instead of the fan.

You need approximately 5 feet of space on all sides of a pool table. Without dimensions of the room showing windows, fireplace, passage way it's difficult to give any advice also given there's lots of clutter in the room. I can see a living room behind the fireplace so I'm presuming he doesn't want any seating in the pool room.

I also don't know what the pool table looks like...

Use art to cover those square niches. Less is more in this space. No shelves, they just collect clutter, but a small cabinet for wine, liquor or a coffee bar would work best nearby to clear some space in the kitchen.

IMO start with much better storage in those spaces a nice large pantry cabinet where you have the bookshelf would be great . It would get the clutter off the cabinets and give you a place for all the stuff for the pool table too. As mentioned the space needs to allow for5' all around the table so a pretty large space . You need good lighting that is the reason most pool tables have 3-4 lights above them not a single chandelier .We did our space for the pool table sort of fun retro style .

I would paint the room a dark rich color of choice.

Edit all the furnishings to only what you need. The kitchen doubles as a bar,

bamboo blinds on the kitchen window for texture.

For the Large window, drapery panels the same color you choose for the walls... Denim if you go with navy?

a large area rug to ground the room (an cover the builder grade floor tile)

Interesting backsplash tile, like mirrored tiles... a dramatic ceiling fixture and/or industrial looking ceiling fan. Use all your cuppies, niches and ledges for display... decoys, taxidermy, sporting memorabilia, etc.