Kinect sounds like a genuine good answer. Also Mass Effect, thinking about it, is not horrible. It may be tough to begin, but I think it is actually a fair enough introduction to how mainstream gaming works?
Will you be helping her as she plays a game like the original Mass Effect? If you start her with the original Mass Effect be sure to explain stuff like leveling, the inventory and the iffy check point system. She's going to die in combat and lose hours of progress due to its horrible auto saving if she's not being coached to manual save before every encounter. Too bad there's no Story Mode in ME1 for the casuals.
IMO, if she's never played a TPS or FPS before, you might want to steer her towards Portal first so she can get the concept of the way those games control. You've got your look, move, jump, crouch and portal commands. Pretty simple. There's no sprint, grenade, cover, aim, weapon select, squad commands, inventory, health, buffs, biotics and etc to worry about while she gets a feel for the dual stick movement. For some people, learning to move and look with a controller is like learning how to ride a bike for the first time. Mass Effect might be too complicated to control as someone's first core game.
