There hasn't been much suggestion of Double Fine stuff, it might not be a bad idea to look at their catalogue. Grim Fandango strikes me as a possibly-great choice, also Costume Quest and Stacking. Maybe Psychonauts, depends how she is at platforming.
I'm always willing to throw text adventures out there, although they require a certain mindset. Still, I will
always happily float Photopia for people interested in narrative in gaming. And, as others have mentioned, while it's not a text adventure Her Story fits nicely into that bracket (Sam Barlow has roots in
text adventure dev)
On the subject of narrative, what about the Inkle games, particularly 80Days? And on a similarish note, might be worth glancing in the direction of the Fallen London family of games, especially Sunless Sea.
RPGs feel like they ought to be a good fit, but finding the appropriate balance of complexity for a new player might not be trivial. There's Costume Quest as I mentioned beforehand. A Paper Mario or a Mario & Luigi strikes me as a good fit, but there's no simple way to get to those. And for a left-field suggestion, how about Fable Anniversary on PC? It's been a long time since I played it (back then in the original Xbox form), but I don't recall it being overcomplex.
Wondering about something simple in the Diablo vein. Deathspank, possibly, but it's not for everyone, the jokes are a bit heavyhanded. Would Torchlight fit the bill, or is that too complex? I've not played Titan Quest or Van Helsing, and not made proper inroads into Dungeon Siege, but I imagine they're all possibilities as well. Narrative isn't great in these, though.
I've been trying to figure out if there's something open world that manages to sustain interest but
isn't full of complexity - that is, something that rewards exploration. BOTW is the obvious fit, but possibly complex, it's hard to be sure. That said, it does help that the key interactions in BOTW are very intuiutive. I'm actually thinking Wind Waker would work, but again, that doesn't currently fit your available platforms.
I know some people who aren't deep gamers who have warmed to Assassin's Creed for the atmosphere, but you'd have to judge if the controls would be too overwhelming. It's a game that gets the imagination going, though, and that can draw people in.
