What I really love about this is the softness of the mouth and muzzle area. It's amazing how she has captured that which is so true of horses. I think the picture has a lot of sensitivity actually. The yellow ear covers slightly over-dominate and the way they are painted - very thick and textured - makes the eyes recede into the background and eyes are the thing that are so important in portraits whether of humans or animals. Although the eye is a characterful kind eye. But is knocked back by the yellow.
I do admire her going for it with the yellow ear covers though because she's trying something. She is trying to capture a different texture by using different paint thicknesses. This is the sign of someone trying things and engaging with the materials, not just someone trying to make an image.
For me, the left side is less successful, but the main face and muzzle in particular are wonderful.
Basically pet portraits are quite generic and tend to have a particular style and look to them. There are some people who are absolutely incredibly at pet portraits. But it can also be a bit of a straitjacket artistically. There is an expected learned style which people can get stuck in and find hard to branch away from. There are so many tutorials online about how to do fur (for example) for the expected pet portrait. It's not necessarily that individual.
I think if she could take the sensitivity she has got in the way she's captured that soft mouth area in this work and apply that kind of specificness/particularity to the rest, this piece could be really stunning.
You say art is not where her heart lies so maybe my thoughts are not very helpful or relevant. She's lucky to have a lovely skill in addition to other things in this case. If she was interested in art, my advice would be to block her ears to the sniffy people on here. At her age, it's a lot about levels of exposure to different materials and how much confidence is encouraged to try things, as well as basic drawing skills. Being exposed to lots of different artists and artistic movements. Trying out different ways of doing things and different materials. Art is about experimenting and also being bold enough to risk things toppling over into failure. The trouble is our exam culture is not exactly brilliant for encouraging these sorts of qualities. It's a good exercise to think what is it that I bring to painting that others don't? What is the thing that I particularly have a feel for? I'd say it's a real sense of sensitivity in this piece, that you don't see in all pet portraits. A lot of people can draw, a lot of people can draw extremely well. But it's those other qualities like sensitivity or specificity in the way you look at a particular subject, that makes the difference between a good drawing and something more.
£15 is ridiculously cheap. You have to think is it worth her spending 6 hrs for such an amount. Plus as others have said, commissions can be difficult and stressful and people can be ridiculously demanding even when not really paying a proper price. She might want to think if she does some finished paintings she really likes about printing them on nice paper with a nice sense of the personal about them to make semi-handmade cards? If she works in a tearoom, often cafes welcome art to display on their walls. It might be they would be open to displaying some for possible sale, or cards. But @Stainglasses has a good point. Trying to monetise a hobby can take the pleasure out of it. So that is worth bearing in mind. Love of drawing and painting can develop and deepen across a life-time. It would be a shame if it became a chore for her so young.
As for @ArseInTheCoOpWindow's teaching in the "best art department in the city" whatever that means, what kind of teaching produces a list of criticisms but no actual analysis of what is good and what can be developed further? Art is not just a set of "nots" like avoid black and not enough highlights. Really good teachers encourage students to really look, challenge their preconceptions, and really engage with the subject, materials, experiment and have confidence. Really good teachers inspire people to go further and better. An uninspiring list of "nots" is no use to anyone.
Good luck to your DD, OP! She sounds like she has many talents and interests. Encourage her to keep drawing no matter what else she decides to do.