If you'd like some simpler zombie action, Resident Evil: Umbrella Chronicles is a nice, easy-enough game to give you both some thrills while not being too overly difficult. I really enjoyed playing it with a variety of friends and even my fiance, who is terrified of certain more realistic zombies and plays almost nothing but RPGs, had fun with it. You get a quicker glimpse at the overall Resident Evil story, while testing your skills in an on-rails shooter just like the arcades. Lots of levels to play, a variety of weapons to pick from, fun destructible objects all over the place, and of course lots of different zombies and monsters and hidden secrets.
As a completionist and hardcore gamer, I like how well-hidden the secrets are and how even after 5 playthroughs of half the levels, I haven't gotten all the stuff in most of them and they're still a blast to play. It's neither too easy nor is it wall-punchingly hard on the harder difficulty setting. If my variety of friends who have played and loved it amount to anything, Umbrella Chronicles (and probably Darkside Chronicles, but I haven't bought that one yet) is a great game to get for co-op play.
I have probably enjoyed it as much as or even more than most of the main RE games.
Boom Blox may not be what you're looking for so much, but it's another great multiplayer Wii game. It's competitive, but the fun kind rather than the wanting to strangle each other kind. And players with less experience don't really need to worry about failing miserably against everyone else. Physics games (mostly in 2D usually) are a bit overdone but this one strikes that perfect chord where strategy is key, luck plays its part, anyone can pick it up and play easily and it's all as satisfying and fun as that one part in every Jenga game where the whole tower comes toppling over. I would recommend 4 players or at least 3 if you can get them, but a game of Boom Blox really tends to draw a crowd. It's one of those good party games where you start playing and even the people watching are having fun.. and often wanting to join in.
I'd say Sin and Punishment Star Successor, since player 2 just has shooting to worry about and doesn't have to dodge and do melee attacks and such, but you would be dying a lot (everyone does. The game is brutal!) and it might turn her off from the game.
New Super Mario Bros Wii is one of those "duh" ones. Single-player, it's a challenge. Multiplayer, well, you don't exactly magically get good at Mario by having someone else with you but the game is light enough on multiple players that it never feels too punishing, yet not too easy either. You can mess up a lot in multiplayer and probably end up using lots of continues, but it's never like you fall off the cliff once and that's it for the rest of the level. As long as you have an extra life or the remaining player(s) collect 100 coins, you come back within 5-10 seconds.
That and Mario Kart Wii. Simplistic controls are key to introducing anyone to games, and turning the remote to steer couldn't be more simple to understand. Everyone I know plays it that way, too, simply because Mario Kart is just one of those fun games where luck is involved enough that the precision you might lose from the control scheme isn't bad enough to ruin the experience of moving around like you're actually steering. Maybe it looks silly and takes more effort, but it's not about winning, really.
In general, I'd just steer clear of games with overly complex controls. Anything requiring two analog sticks to be used at once is a no-no, unless it's along the lines of I MAED A GAEM WITH ZOMBI3S IN IT!!1 or Galaxy Wars. My fiance isn't the only girl gamer who can't aim and run at the same time in a Halo match. Resident Evil 5 would be way overboard, as I'd imagine, since you're not only aiming and running at once, but doing so in 3D, with lots of fast zombies kicking your ass and lots of other buttons to worry about pressing. It's second nature to you and me but to a newcomer, the two sticks alone can be a daunting prospect. It always surprises me how some people just don't realize that at all. Controllers are SCARY to new players.
Most of my suggestions are Wii games, but it makes sense. The Wii is an extremely friendly controller. One big button that does main-button stuff. Trigger on the bottom. 4-way pad. One analog stick. Trigger on the other hand. Able to point at the screen, intuitively. Shake gestures. You give your girlfriend a remote and nunchuk and say point and shoot. Shake to reload. The other hand is your knife. To a gamer that might seem silly and unnecessary but you'd be surprised how much easier an intuitive control scheme like that can be to a newcomer.