Love the new style windows. It is usually suggested to decorate at a normal ceiling height. Using the bottom of the upper windows as the height to decorate. I would not put wainscoting on the window wall.

Paint the ceiling

If you think of the ceiling as another blank canvas, a lot of possibilities open up. You can create a “wow” factor in a room by the ceiling alone, but even if you just want to do something subtle, painting the ceiling can help lower the ceiling for a sense of coziness while also highlighting the beauty of the tall ceilings. Go for a few shades lighter or darker than your walls for a subtle effect or a bold color for something more dramatic.

Do two-tone walls

One solid color lengthens while color blocking shortens. There are many options for two-tone walls such as doing one color on the top and another on the bottom.

Accent with tall plants

Strategically place tall, potted plants to fill up some of that empty air space and make it feel more inviting.

Warm up the windows with curtains

The soft texture of the fabric will finish off your windows and draw your eye lower so the space feels more intimate.

Create intimate conversation areas

Avoid keeping the furniture far apart, as this will amplify how large and cavernous the room feels. Instead, pull the furniture in closer together and anchor it with an area rug. If the room is particularly large, create a few small conversation or activity areas with a natural traffic flow between them.

Use luxurious textures

Luxurious textures can always be counted on to bring comfort and coziness to a room. Instead of using a lot of cold, hard, shiny surfaces, incorporate things like wood, faux fur, and plenty of soft textiles.

Instead of adding another north-facing window (which isn’t going to bring in more light since there’s no light to be brought in when you’re facing north!), they’d probably be better off adding skylights.

A window above a fire place just doesn't seem quite right for some reason, even though there's obviously no chimney. What about a lovely big mirror instead above the mantle? A mirror can add almost as much light as a window.

As suggested just ignore the height, decorate with artwork at normal height, I see the chandelier looks high, if it is, lower it to a better height. If you want to add a mirror make shure it reflects something nice at different angles.

You daughter has painted the wall an accent color -- different from the surrounding walls -- which tells me SHE doesn't want to minimize it. She might even want to add a large sunburst mirror or other (sky) metal interior wall sculpture.

https://www.touchofclass.com/evening-sunset-wall-sculpt/p/Y362-001/?code=DMF03&color=&size=&tid=DMFPLA&gclid=EAIaIQobChMIkcbcuPmZ7AIV-QiICR2_pQuqEAQYBCABEgJnJ_D_BwE

@C Marlin.....the chandy is not necessarily at its final position. They have the chain and wire looped around a bit so they can adjust when ready (it’s on a lift btw).

@Helen 72.... Although I don’t think they are not big on mirrors, I will mention it.

@suezbell....the paint on the wall is the original color from the former owners. They’re waiting until all the trim work is done before painting the room.

If she wants to minimize the wall, paint it the same color as the rest of the room.

They could embrace it by using art such as this wall sculpture. Here six metal wall sculptures were “strung together”. I also agree that once ( if ) drapery is hung at the lower window’s height this may become a non issue. Expensive option : would they be amenable to adding a stone wall above the fireplace ?

following

I would add color to the room.