I'd say the biggest difference is the amount of texture, or noise you have on your work, in comparison to your friend's work.
Look for ways to simplify the volumes and shapes on your tiles, and then she how can fit them to match your friends take on it.
one thing, in terms of color is, you'll notice your friend doesn't that much contrast on the same material. sure the water contrasts with the dirt, but the dirt elements don't contrast that much with each other, which makes them look flatter. there's still texture, but it feels simple.
on your textures you have black with almost white pretty close together. that just makes it a very different approach in coloring.
