There are several Sudoku games on DSiWare. I own them all.
Unfortunately, none are as good as the ones in Brain Age 1 & 2. The interface isn't as nice and they all seem to have their own little quirks--one doesn't let you use the stylus, one only has room for 4 temp/note numbers and one doesn't even support temp/note numbers! Even the nice gimmicks--for example, one will highlight all the cells with the same number making it easy to spot what you're missing--don't really feel any better than the Brain Age games.
I don't have any experience with retail Sudoku games, but I'd be willing to bet they're all very similar to the DSiWare ones--they all pale in comparison to Brain Age. (How sad is that?)
Honestly, I'd recommend your mom just got back and play Brain Age 1 again. Unless she's able to memorize the puzzles she should be able to replay the games just fine.
Also, like everyone else said, Picross DS and Crosswords DS are fine choices if your mom is looking to branch out from Sudoku. I've sunk close to 200 hours into Picross and I still haven't finished all the downloadable puzzles. The interface is fantastic. This game is an unbelievable value at $20. The core game play is a lot like Sudoku--logic-based, tile puzzles.