If he is still running the Stock filter he needs to get rid of it ASAP. They are junk. Tell him to get A TWIN AIR set-up. It costs about $80 but well worth the money.
Pipes: Yoshi and HMF seem to be the favorite. I had a HMF and it was way too loud. 102DBs! So I am going with a Yoshi now. Its about 98DBs without the quiet core.
But with a pipe you need to run a Fuel controller and thats another $200 at minimum if you find one used. After you buy a pipe and EFI controller your going to have the same cost as a VDi and a VDI will make more power then the pipe and controller. Now if you can get a pipe and VDI then you have something there lol.
Another option is clutch kits. A yellow EPI secondary spring and 20 gram weights should make it have some great low end. But will scarafice some top end