Impreza help...need new battery or alternator?

I am posting this for a friend;
He went to start his car (1995 or 1996 subaru impreza), and it started fine, then the tach fell to 0, allthough the car was still running. Then it died. Tried starting it again and the engine would just click over repetitavly, like a toy machine gun. We gave him a battery jump, and it held fine for about a minute, then died. So now I am thinking that he needs a new alternator, because it couldn't hold the charge. But could it be the battery? All the belts are fine, so I am thinking we need to replace the alternator because the battery could not cause the tach to jump like that then fall to zero while the engine is still running could it? Can the alternator cause the tach to jump around then die like that?

BTW whats a rough estimate for a replacement alternator if thats the problem?

What else could be causing his car to not start?

Did you check to see if any of the cables going to the battery or alternator have come loose or worn through anywhere? I had one fall off of my battery once while driving, all the guages dropped to 0 and after I pulled over and tried to start it it only clicked. Saw the cable and put it back on and it ran fine. I'd bet on that being the problem.