Well, actually, under most leash laws, a dog being in his own unfenced front yard off a leash is still a no-no. Don't know what the legal situation is where you are.
But, what they did was equally wrong, if not more so, by being away from home, on someone ELSE'S private property, with a dog off leash. I know it's a rural area, but still....
If my memory of such legal things serves, the people aren't "trespassing" unless your property has posted signs or you tell them to leave. That still doesn't make it right.