Oh jesus, DS had one of the hugest libraries ever, especially given the easy access to GBA stuff.
All the pokemons - most notably Heart Gold and Soul Silver, which are quite a bit better than BW/BW2... although, if you have a 3ds, skip this step because XY is the best gen to date
Scribblenauts and Super Scribblenauts - puzzle games where you conjure up any item you can think of to solve each level
Brain Age/Brain Age 2 simple arithmetic, counting and word puzzles to keep your mind sharp
Multiple mediocre Castlevanias - Dawn of Sorrows, Portrait of Ruin, Order of Ecclesia - worth a play if you're super into castlevania, nothing to write home about if not (although the GBA title Aria of Sorrows is commonly lauded as the second best castlevania game, so you can give that a shot if you want)
Etrian Odyssey - REALLY FUCKING HARD roguelikes where you draw your own map as you go along, definitely not for everyone
The incredible 3d remakes of Final Fantasy 3 and 4 (seriously, do not miss these)
Chrono Trigger (remake, obv)
Super Mario 64 DS - definitely a solid title, offers play as multiple characters instead of just mario
Phoenix Wright - humorous pseudo-court franchise in the style of older adventure games
Zelda: Spirit Tracks (skip phantom hourglass, it was terrible) - very very smooth stylus controls, great dungeons, kinda tedious map travel (you have to drive your train everywhere, which gets old as soon as you get bored of having a cannon mounted on your train)
Final Fantasy Tactics A2 (DS game, following FFTA1 on GBA - both are pretty solid Turn Based Strategy with lots of optional content)
Kirby Squeak Squad, Kirby Super Star Ultra, etc (if you're into that sort of thing)
And my personal favorite... Osu! Tatakae! Ouendan!
Ouendan was a japanese rhythm game that spawned the slightly inferior, westernized version called Elite Beat Agents, but the original japanese game was full of some pretty great jpop/jrock. It's literally all in japanese, but you can figure out the menus without too much hassle, and the gameplay is all stylus controlled, hitting areas of the screen in sync with the music. Very addicting, very fun. Ouendan was followed up by Elite Beat Agents, and then Ouendan 2, but the true gem (imo) is still the first game. Well worth a try.
Have fun with your DS mate!