Unfortunately, I don't think there's anything you can do to prevent further deterioration ... Only thing that comes to mind is a coat of lacquer? I am just guessing though ... Zamak usually cannot be replated, and especially not once it reaches that condition.
That razor is too far gone for my suggestion, but I coat my vintage Zamak razors in Renaissance wax. The razors remain usable, the wax is virtually undetectable, and it might add a few years to their lives ...