You guys are not being helpful. He's already stated that he is friends with these people and wants to continue to be friends. Try to help him, not give him your prejudices.
An electric fence is a good idea. Do you have access on the other side of your chain link fence? If so, run 2 strands a couple of feet away from your fence, make it as "invisible" as possible. He (the dog) won't be expecting it so it won't be hard, just don't put flags or anything. As he runs to your fence to start digging he will hit the electric fence and think god got him.
A friend of mine had goats and she ran a one strand electric fence using the fiberglas short posts around her yard to keep them in. I visited her just after she had put it up, I had my dog with me, didn't even think about Lisa hitting it. She jumped out of my car and went to see the goats, hit that fence, yelped and took off like a bat out of H. My friend was way out in the country and I spent the next 4 hours going to every neighbor in a 4 mile radius looking for my dog and leaving notes. I got her back the next day when someone over a mile away called me. It was years before she would get out of the car at my friends house. She was a weim.