Lara Croft and the Guardian of Light is played Diablo style, needs two people to work together (Croft shoots a grappling hook and the indian walks on the wire to get someplace and stuff like that) has a guy and a gal, but there's shooting, blowing things up and noise, so it's not for girls who don't like a little mayhem.
Archon can be played vs or coop against computers. It's chess like, easy to control but it's also a very simple gaming experience, with no story whatsoever on your coop except eliminate your (computer) opponent.
Not so sure she'll like either. Her games seem like she enjoys a little "cute" on them, just like my wife.
Tidalis is like tetris and has coop but the control scheme for the second player is harder, since using the mouse is the easiest.