Ju-On: The Grudge!

Metroid prime trilogy. lol I kid, try ninjabread man, I know I know it's a terrible game but i've heard it's a peice of toast.

Cooking Mama. :D

She's a woman after all.

My wife loved to play Animal Crossing. She was addicted, and she can not stand games that have complicated controls. I would also suggest as possibilities: MarioKart, World of Goo, Boom Blox, Wii Sports, and Endless Ocean.

Hidden object games (Escape the Museum, The Vanishing Files) might be the go. They use just one button and pointing. Along those lines, an adventure like Broken Sword: The Director's Cut might fit the bill too.

All of the hidden object games are really cheap, and while the titles are 'easy' to control, the puzzles in them can be challenging. I've also heard that the hidden object genre is a big hit with mums.

wii series and nsmbwii

Of the games I've played I would have to nominate Wii Play as being the simplest and easiest to pick up and play but then there are some games like Tanks which require a little more skill. Basically most of the games are very easy to learn but there is a bit of range for when your Mum starts to acquire some skills.

NSMBWii might be an option if you are playing together but depends on how quickly your mum can learn the basic mechanics. i.e. is it going to be before or after she gets frustrated with the game.

world of goo (a kinda physics/puzzle/building game where you only use the ir pointer and the a button).

also, are easy to pick up but hard to master games what you are looking for? (simple controls, but difficult to master sometimes fast-paced twitchy gameplay, like nsmb wii, mario kart, tetris, wii sports/resort, ect.)

Well, there are 'tutorials' for the first hour or so, but they're nicely woven into the gameplay and use very simple language.  The controller layout is very simple as well--all you need is the Wii Remote, using the pointer and the A button (with the B button optional if you want to run).  The other buttons do things as well, but you can also do those things by pointing at icons on the screen.

Animal Crossing is an incredible series, and I wholeheartedly recommend it.

This:

less than £10 /$10

Everyone I know who has it seems to love it and spend hours on it.