The most common term for this expression these days in the USA is to be "laid back". She is so "laid back". It means that no matter what happens, she almost never becomes upset or stressed. Sometimes someone can react without much feeling. This in pychological terms can be called "flat affect". Flat affect can be a sign of depression or disassociation (not feeling connected to events and things around you). But mostly, someone who is "laid back" is simply very "low-key" or "easy-going" (other words that mean the same thing). There is nothing wrong with them, in fact many people would like to be as "relaxed" as they are about life.

He's down to earth. ^^ (British)

You could also use: level-headed together (informal) sedate relaxed unfazed

A cool customer ;)

As cool as a cucumber A veritable Mr Fonz A real Mr Ice Cool Mr McCool from Coolsville, Tennessee Coolio So laid back he's horizontal