I really don't think you should have to pay any money to re-instate your supply to be at least as good as it was before. Understandably, you don't want to fall out with your neighbour, but perhaps he needs to pay a different plumber to fix your supply to be as good as it was before, and make his garage connection to his own supply.
If you both really want to have lead pipes replaced, then at least split the cost.
If you both really want to have lead pipes replaced, then at least split the cost.