Your dad will have an easement for his pipes, wether written into his deeds, or simply because it has been in place for over 20 years.
He is very unlikely to be obligated to pay anything, and what's more is entitled to the drainage routes he has and they should not be interfered with without his agreement.
If the neighbour wants to change things, it his cost and your dad should get detailed plans and assurances that at the end of the works he still has the same service. Solicitors need to be involved to draw up an agreement, the neighbour should cover all costs including your dad's solicitors fees.
I am no expert, for some free advice from some well informed people post your query on gardenlaw.co.uk.
Cheers. Stu