It seems like she enjoys straightforward/simpler platformers, so here's what I can come up with in that vein (these are all gonna be Wii games)-
Kirby's Epic Yarn has co-op- if she liked the platforming/art style of Rayman Origins but it got too hard towards the end, this might be a good way to go. You literally cannot die (all that happens when you fall in a pit is you lose your gems) and if she gets stuck on some platforming, in co-op you can pick her up and throw/carry her past the tough spots (it can get a bit tricky at the end). Plus, it's SO cute and super charming and really relaxing to play. I love the shit out of this game and would highly highly recommend it.
The New Super Mario Bros games can be fun in co-op, but not so much when there's a big skill disparity between the people playing- the camera follows the person who's farthest ahead, and if they get too far ahead the person behind will get scrolled off the screen and die. Plus, the levels can get pretty tricky. Might be worth looking into, but it's probably not a good fit.
Super Mario Galaxy has "co-op"- a second person can pick up a controller and point at the screen to collect things and stun enemies for you with a cursor. Maybe consider picking this up and swapping the main controller back and forth between levels?
Those are the big ones that are coming to mind at the moment. I'll come back and edit this with more if I can think of any.
Also, don't forget about the Virtual Console or that the Wii can play Gamecube games- I don't have any specific recommendations there, but it's worth taking a look to see if there's any old games she might like.
EDIT: Speaking of Gamecube games, maybe look at Legend of Zelda: Four Swords Adventures? You'll have to get it on disc, but I remember that game being pretty fun.