Showing when heads up all in?

[Beezos]

In a cash game, what is the ruling when you are heads up and one personal is all in? Last nite, I played with this girl at a local poker club, and she would not show unless she won the hand. Two of three time she lost and mucked. The other she showed her cards when she spiked a third jack to beat two pair on the river. Is that considered slow rolling? Is that against the rules or just poor etiquette? This chick was tough as balls, and liked to talk alot. She had some brass tackle. Definitely, an intimidating player. I dont know if the not showing part was part of her game and image etc... Any thoughts?