I would write them a letter/email or even talk to them
Dear so and so..
"thank you for being so compliant about the incident, it was very nice of you to wait with us and come with us to the vets, but id like to talk about how we can make sure it never happens again.
I love my dogs very much, and If this ever happens again, We might not be so lucky.
Please, keep your dog on a leash and under control when hes outside, or build a fence around your front yard, it would ensure that something like this never happens again. I would not feel comfortable living next door with your dog allowed to be off leash in your front yard, please remember that my dog was the one leashed and on our own property.
or something like that
I like this idea.