The first part here is one fact to consider, the second part is my opinion.
Fact:
Does he have any Game Boy Advance games? If he upgrades to any other DS made past the Lite (DSi, DS XL, 3DS) he will lose the Game Boy Advance slot and not be able to play those games. Once I get a 3DS sometime in the future, I won't be trading in my DS Lite because I don't want to lock myself out from replaying older games I already own.
Opinion:
I'm not sure I see the wisdom in spending even more money to upgrade from one obsolete system to another obsolete system. The 3DS is what Nintendo is developing and supporting right now and since he already has a perfectly functional DS I don't see the point of spending more money for something that isn't better and isn't even new.
Full disclosure: I hate the DS XL. I think it's way too big to be easily portable and in my opinion, the graphics look stretched out and bad, like they just took a picture and blew it up and they didn't care if it looked pixelated. Does he want it for any reason beyond "it's bigger"? Just thinking he should have a look at one in the store to see the graphics and the feel of it in his hands before making a decision.