Manufacturer not replacing broken window

Two years ago I had new double-glazed windows installed. Was pretty happy with them, until a few days ago I heard a loud bang while I was in the other room and I went to see one of the windows had spontaneously shattered. It didn't appear like it was hit or anything. It was a really hot morning with the sun bearing down hard, and my best guess is that some kind of thermal fracture happened. (For reference, this is in New Zealand, it's summer here now.)

So to me it doesn't feel like windows should just spontaneously shatter after just two years, regardless of how hot it is. So I contacted the installer. Initially they gave me some bs like "happy to give a quote" bla bla. And they said it wouldn't be covered under warranty. In NZ we have the Consumer Guarantees Act which obliges manufacturers to replace goods if they don't meet typical standards of being fit for purpose. I quoted this to the installer and then he replied he could have the window replaced for $300 for the glass, and not charging for labour. So I contact the manufacturer, and they reply with the usual: not under warranty, contact the installer.

I don't know what to do now. Should I take the $300? Even though it's not that much money, still kind of feels wrong to get charged for something that seems like a manufacturing defect.