Well, if his goal is to gain experience working with an embedded platform, I don't see any reason why working on homebrew games would be a waste of skills. Any skills he picks up there will transfer over to any other portable platform, such as working within memory constraints and on odd systems (DS's two screens and 2 processors). I recommend everyone do a bit of embedded device programming at some point, even if you don't do it professionally.
The DS's life as a commercial system is reaching near it's end so I wouldn't choose now to go into commercial DS development as it is. It's also fairly difficult to acquire an official DS SDK even if you are a semi-professional game developer with published games. As a result I do not have specific information on the DS (Nitro) SDK.