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You really can't do anything if the owner won't train their dog to behave. The dog will definitely learn it's lesson if it get's kicked. If the dog gets hurt it's her own fault for being irresponsible.

What kind of dog is it? Some breeds have the "herding" instinct bred in, and it would be hard to train that away without considerable effort- effort the owners don't seem to be willing to put into it.

The nipping at the heels is what makes me ask, because that's a herding dog behavior. If they're not willing to train their dog, I'd ask them to keep it either penned or leashed when the horses are around, if that's a possibility. What exactly is the situation? Are you, or someone else, in a position to "lay down the law" about this?

If they really don't want to train their dog, I'd write up some kind of release form (they require these in my home state for anyone partaking in Equine activities like riding lessons or trail rides. Any "business".) saying that they're taking full responsibility for their animal's welfare and that if it gets injured by a horse, it's not your fault. They make stables and barns post signs around the property in my state saying that by coming onto the premises, you accept reasonable risk of injury due to the nature of the animals and the situation.

However, comma pause, does your area not have leash laws? Everywhere I've been dogs that weren't in a yard had to be on a leash, and I doubt the only riding you're doing is in their front garden.

Then I'd get their phone number, and just call them ahead of time and let them know when you'll be coming through and ask that they put the dog in the house or on a leash.