It does not say how the trespasser was behaving. If he was negligent then yes I do think the owner has the right to sue for the current value of the horse if unable to be used for her intended goal, however if recoverable than only the medical costs.
I do not think however, they have any right to sue for "future possiblities" of the horse. As the horse hadn't been shown in any sanctioned shows in two years it is hard to say that it was currently a valuable showjumper. I'd like to believe that all my future foals will win big awards, but no guarentees in life.
Also suing for costs of missing high level shows... why... she could of tripped and twisted her ankle... gotten a cold... horse decided not to load up that day.
Horses are a dangerous sport... that's why they post big liability signs everywhere stating that very fact. You get on a horse, you risk your life. I don't care if its a youngster fresh off the track or an old been there done that horse. They are animals and not machines. If you want to ride them you choose to put your life in a precarious position.
Was the guy right to be on her land... no... but is he accountable for everything that happened... no, with a condition. If it was his intention to cause havoc then I think he should get a severe penalty.
Life happens... if you insist on being put in bubble wrap... pretty soon you won't be able to move.