Well if you catch the guy doing it again, don't talk to him see if you can find a police officer on duty, and ask if the officer is willing to follow you to the place the dog is located, then show him the proof that is not the first time they do that ,that will take care of those bad neighbor's and probably they even take the dog away from them.
The low down is,
If they do seize the dog..
She's gonna wind up on a shelter for adoption and her chances to survival are low,
But if shes a pure breed lab and as sweet as you mention it, she may have a good chance to be adopted,
so you have a tough choice..
You can try to adopt her but I don't think the neighbors are gonna like that.. ha-ha!.. :P
Or If they really apreciate her, they will go to the shelter, pay the fine and bring her back home with them...
I don't think they will try to do it again unless the wanna see the cops knocking at the door every single time they do it again.. :)