I'm surprised that if they've integrated themselves into your flock you don't have an issue with them going to your coop to roost. It sounds like a fence isn't wanted by you or your neighbor so it comes down to each owner minding their own chickens. It sounds like the owner with a containment issue is your neighbor. I know that you have a semi attachment to his chickens. I understand this as my neighbor has a small flock (3) and I probably know his chickens better than he does as his coop sits next to my shop. Still, they are his chickens and their care and containment are his responsibility. Your neighbor needs to accept that if he is going to allow his hens to free range onto your property that they will be bred. If he can't handle that he needs to revisit his chicken keeping practices.
* I agree that with two hens unwanted chicks are probably not an issue unless he wouldn't notice one of his girls setting a nest for three weeks.
* I agree that with two hens unwanted chicks are probably not an issue unless he wouldn't notice one of his girls setting a nest for three weeks.