Guarantee on drilling projects

What is the general laws concerning guarantee on drilling projects in Nigeria? 

In August we sponsored a borehole for a local community in Nigeria. 

The contractor drilling the borehole was the same guy whom had committed a geo survey beforehand. 

Now, half a year after the inauguration of the water service the borehole has collapsed with the pump inside. 

Allegedly he casing that has been used has been to weak. 

The contractor offers a discount on a new borehole, but I'd say he should take full responsibility and cover the new borehole and pump entirely. 

Has anyone been in a similar situation? What did you do? 

Does someone know if there are laws that'll back our claim?

Best regards
Charlie