Let's be honest, if he joins with the 1,000,000 HKD "joining package", it's doable.
It's just enough to cover rents for a 700sqf flat in a remote area, or a 300sqf in Central for 3 years.
Biggest issue with a family: schooling. 3+ years of waiting list for Kindergarten or Primary in International Schools. Secondary school less of an issue.
Food can be cheap if you eat in McDonalds or Chinese food most of the time.
If you really want to live in Hong Kong, then Hong Kong airlines is probably a better deal (housing package, FO rating, and they are even more desperate than CX to crew their planes)
But most of the other issues (schooling, pollution,...) remain the same.