You should consider digging a trench that would re-route the leak away from your property and then get the repairs done. Find out who his insurance company is and file a claim against his liability coverage. Try to do that before doing the repairs, though you could re-route to protect your property from further damage.
The neighbor is a real pain. I have had to call animal control on him because of animal neglect. I have had to call the county health dept on him for garbage (a cattle hay feed ring filled to the top of it with garbage - they are about 8ft across and about 4ft high - it was cleaned up by him after the county got on him but it took a year for them to take him to court)
The middle aged man is a real lowlife. He gets employment and then he fakes injuries to get on workman's comp. Then he sues the employer. The county sheriff's dept is always coming to my house looking for him so they can serve him a summons to court for something he has got a loan on and not made payments on.
I went to his door one night last winter because one of his little short haired dogs got out of his house in the freezing cold. He answered the door with a shotgun pointed at me. He quickly put the shotgun down when he saw it was me. That was the end of me going over and knocking on his door. I was pissed and scared at the same time after that.
So my best bet is to start legal action on him and try to get a lien put on his property. We will put in a trench if he does not repair it within 30 days after our attorney contacts him. This has been going for about 6 months and we have been nice about it but we are fed up. I guess I could make a claim with my insurance company and have them go after him.
BTW... My yard is 5 acres and his water well is right on the property line at the front corner of my land and his. It is annoying and if it goes on for another 6 months the damage will be much worse.