My wife and I play a LOT of PC games together. She is not great at games, so easy to learn, not frustrating games are important. Great recent example would be Diablo 3- perfect for drop in, drop out, easy difficulty, can control difficulty using AH for better items, teleport to your friend whenever you want, etc.
Other examples:
Rayman Origins (bubbling out during co-op means that the more skilled player can progress the game)
World of Warcraft - higher learning curve for a new PC gamer
Lara Croft and Guardian of Light - a bit trickier because the two characters have very different skills, so it's hard to explain what she has to do at any time
Magicka - this is brutal and unforgiving, not recommended
Nation Red - fun timewaster
Also you said she liked PvZ.. if you have an Xbox 360, the XBLA version has a really cool co-op mode for the whole campaign (also you could 'co-op' it on an iPad).
I would also stray away from games that require WASD and 1st/3rd person camera controls for a beginner, this is a really big learning curve for some people.
Other examples:
Rayman Origins (bubbling out during co-op means that the more skilled player can progress the game)
World of Warcraft - higher learning curve for a new PC gamer
Lara Croft and Guardian of Light - a bit trickier because the two characters have very different skills, so it's hard to explain what she has to do at any time
Magicka - this is brutal and unforgiving, not recommended
Nation Red - fun timewaster
Also you said she liked PvZ.. if you have an Xbox 360, the XBLA version has a really cool co-op mode for the whole campaign (also you could 'co-op' it on an iPad).
I would also stray away from games that require WASD and 1st/3rd person camera controls for a beginner, this is a really big learning curve for some people.