Remember that it's not so much a matter of if the drawing is an accurate representation of the reference photo, as it is whether or not the drawing is a pleasure to look at. If no one had that reference photo to go back and look at, they would have absolutely no complaints. The the drawing itself reads very well as a spirited horse with a beautiful brown eyes. If going back to darken that I called is a risk of smearing a bunch of things and ruining the whole drawing, then abort the mission. Just keep the drawing as it is and give it to your friend. I promise you, she will be D lighted.!