Haha, so it Seems Portal 2 is Popular, obvious choice yes- i'm sure i'd carry her through the Co-op, im pretty good at Portal myself xD still a great idea though..
normally my list of games is better than this, but i cant seem to think of too many..hm.
Remember though, broad works too- i mean with some friends i can name many games not strictly advertised Co-op that still turned out to be a co-op experience: Unreal Tournament 3 split screen, Halo: Reach campaign, hm.. Champions of Norrath definitely.. if that broadens the topic any.
Oh and preferbly not PC..my rig is real outdated these days, and she's only got a Laptop that is kindaaa..meh.
^^ Unless they're older games..
As someone kindly suggested one of the BEST dungeon Crawlers ever: Diablo 2.