Actually, horses can get bordetellosis, which is caused by the bacteria bordetella bronchiseptica--one of the organisms most often associated with kennel cough. However, it is very rare in horses and from what my vet book says it's unlikely that a horse will get it unless he is already weakened by some other condition.
There was someone that had a horse diagnosed with this a year or two ago. I was thinking it was someone on here, or a close friend or relative of someone on here, but maybe I'm thinking of a different board. I remember someone posted about it and there was discussion about how rare the condition is in horses, and how it's to be hoped that this won't become a more common horse ailment in the future.
Can this person not come to visit without her dog??? I'd think that if the dog is ill it would be better if it stayed home anyway.