Even down south the calcium levels are much higher than in the eastern states - we use Brita water filters but for a family of five that would be expensive.

It sounds like some sort of high tech filter fitted to the water sources to their home might be the way to go and professional advice sought for that.  If they have small children, perhaps also talk with their medico for recommendations.

They will probably also need CLR to combat the calcium and lime build ups in toilet bowls and calcium crust around the kettle too.