I unfortunately just dealt with this very thing so I can fill you in on what I have learned from others.
The best treatment for ich is to remove all your fish to a separate hospital tank with a filter, a heater, a light, and some places for the fish to hide. Ideally you could use an aquarium or your quarantine tank, in a pinch you can use some Rubbermaid containers and some PVC tubes and elbows to allow the fish someplace to hide.
Once your fish are in a separate tank you can treat them with copper, which is effective against marine ich, or with hyposalinity, which involves lowering the salinity of the water for a period of time (the fish can take it; the parasites can't.) Note that once you've dosed a tank with copper, as others have said, it is very difficult to remove all of the copper, so you won't be able to use that tank for live rock, corals, or invertebrates. But it should still be fine for fish. Your friend's tank may be FO at the moment but I would still not use copper in the main tank as it will prevent him from ever keeping corals and inverts in that tank.
Copper may stress the fish since at high levels it can be lethal. But it is very effective. Hyposalinity needs to be carefully monitored using a refractometer, not a hydrometer with a plastic needle like a lot of people have. Also evaporation can raise the salinity again so with a small tank you have to be careful.
To get your fish out of a reef tank you will no doubt have to remove much of your rockwork. You must get all the fish out of your tank and let the tank sit with no fish in it (inverts and corals are OK) for 4 to 5 weeks. Then the ich that is in your tank will die because it will not have a host to attach to.
If you decide to treat your fish in the main tank there are a number of supposedly "reef-safe" medications that you can use to combat ich. Greenex is supposed to work but it will turn all the silicone sealant along the seams of your tank green. Metronidazole can be used also but its effectiveness against ich is debatable according to many people. You can also try feeding food soaked in garlic, this is supposed to buffer the fishes' immune system against ich but again this is more preventative maintenance, not an ich cure.
I appreciate all the good advice I got here and for my own problem, I decided to go with dosing copper in a 20-gallon hospital tank, and everything worked out fine except for a few fish for whom the treatment was too late and they died in quarantine. The single biggest piece of advice I got was: Don't wait. Bite the bullet, and start the treatment early to save your fish. Best of luck with your situation.
Here's a link to more information on setting up a hospital or quarantine tank:
http://reefkeeping.com/issues/2002-02/atj/feature/index.htm
