[robski]

If you don't have a signed model release form she has you over a barrel. The horse is her property and you need her consent if the photo is being used for commercial purposes. Check out some the stock photography websites which go into the legals in depth.

I don't think saying it's for charity will score you any points.

Yes you can take the photo the problem comes when you try to publish it.

By the way did the model sign a model release form to say the photo could be used and if and how she would be paid ? she could comeback and ask for a share of the profits.