I thought he went from the Modem -> PC -> PS3 so he uses his computer as a router and it passes the trafic from ps3 to the modem.
The PC can't act as a router.
He needs to connect straight to the modem to the PLAYSTATION®3 via Ethernet while resetting the modem and removing the USB cord connecting the modem to the PC if wants to keep his current setup. This setup (modem only) would eliminate him being online with both the PC and PLAYSTATION®3 at the same time, which is why he would want to pick up a wired router. The setup would resemble this:
he could use Internet Connection Sharing in windows to allow it
You can, but I still don't advise it if you don't need to. I would only suggest doing it if you want to connect wirelessly and have a laptop nearby to make it act as a wireless HUB and allow you to connect wirelessly without the use of a wireless gaming adapter or wireless bridge (supposing you're using the PLAYSTATION®3 20GB version without wi-fi).
The easiest solution is to get a router (or stick with the current modem only setup while unplugging the USB and resetting the modem) since it also adds security to your network by forcing all clients to renew their IP addresses at various times, so it's no guarantee you'll have the same IP Address at a given time.
hes connecting wired now so why does he need to make it wireless.
second how does it add security when he will still have a static ip to the outside connection.
A router would allow him to have some control over port blocking but he can use a software and get some protection as well. The only real advantage in getting a router would be you would not need to have your computer on when your useing internet on ps3