Unflappable describes someone who remains composed in the face of adversity or scandal, "impossible to fluster."

Stoic has the same meaning, but often implies indifference to pleasure as well as adversity.

You can say decision-maker. Whilst this might look like 2 words, it's a single term, and because of the hyphenation, it's practically one word.

If you don't need it to be one word, you could reword the second bit to "she calls the shots".