Puzzle Quest, World of Goo

BattleBlock Theater, maybe you could play with her too :3

Manhunt

VVVVVVV or FEZ.

Scribblenauts?
Plant vs ZOmbies?
Trine?

There's this indie RTS called Eufloria
it's relaxing and starts at a very slow pace :D

Amnesia: The Dark Descent
Dark Souls
Dear Esther
Deus Ex GOTY
Don't Starve
Europa Universalis IV
Fear
I have no mouth and I must scream
Postal 2
Spec Ops: The Line
System Shock 2
Thief series
Witcher 2

tiny & big - grandpa's leftovers?

Offspring Fling! ?

Qbeez 2. My mom used to love it.

Edge

Freddi Fish 3

Shelter

The Binding of Isaac ( ͡° ͜ʖ ͡°)

Long Live The Queen?

My mom seemed to like Plants vs Zombies and Draw a Stickman.
She also enjoyed playing those hidden object games, there are several of them on Steam.

Okay guys, I'm closing the thread, thanks for the great feedback! This is the final list I'm gonna show her: Fez, Portal, To the moon, thomas was alone, Eufloria, Solar, Osmos, Strata, World of Goo, VVVVVVV

I entered here for jokes... but... it seems people are nicer than I thought :D. As someone already suggested, bigfish games seems perfect ;)

peggle, zuma

Outlast gg wp bb

Amanita Design games - Samorost, Machinarium, Botanicula
Also, I tried to convince my mom to video games too and she got to like Cogs, Crayon Physics Deluxe and Snuggle Truck.

Manhunt, Postal 2, RapeSimulator, etc... you get the idea.