Cutting the electrical supply to a private well: can I cut this cable?

Question:
My customer does not want her next door neighbour to have the electrical supply to a private well so I have cut the supply to the pump.

The pump is a shared one which supplies both properties with water from a private well. It has been like this for 15 years apparently and my customer, who has just bought the property, no longer wishes for this to happen. The next door neighbour ran a blue arctic extension lead up to the pump house to re-energise the pump which is on a standard 13A plug socket in the well house.

Can I cut this cable?