The laptop would need to have a wireless network adapter of its own (apart from any it may be using w/ its Internet connection). Establish an adhoc wireless connection (w/ WEP) between the Nintendo DS and that wireless adapter. Finally, enable ICS on the laptop's Internet connection.
NOTE: This doesn’t really eliminate the security risk. You’re only as secure as your weakest link. So it’s theoretically possible for someone to crack that WEP connection and minimally eavesdrop, and perhaps even establish themselves as the adhoc peer. Granted, the risk is rather low, but you should at least be aware.