Getting save games back onto the cartridge?

I've heard there are ways to extract a save file from the cartridge onto a PC. But I'm wondering is there a way to get the save back onto a cartridge?
Someone I know has Animal Crossing and has gotten pretty far but their game is starting to mess up and was wanting to buy a new one, I'm just curious to know if there is anyway he can keep his save.

Yes you can, using several tools. If you've got a slot-2 device, you'd want to use RAC for Animal Crossing in particular. There are also other programs that can manage save files for other games, including Eepinator, SavSender, and NDS Backup Tool.