Nissan's useless warranty

In September 2020 our Nissan NP 300 overheated. As it was late and on a desolate road, the driver had no option but to top up coolant levels with water only. We were then told by the Nissan dealership, where we bought the vehicle, to bring the vehicle in. Nissan South Africa together with their insurance company Guardrisk rejected the warranty claim, with the excuse that there was rust in the cooling system which caused the overheating. They further claimed that the rust was caused by the water that our driver added. It recently came under our attention that it was in-fact the head gasket that blew and caused the over heating. Nissan however still does not want to honor their warranty and are looking for all possible yet unfounded excuses not to honor their warranty. The Nissan warranty is therefore useless and does not protect the client in any way!