Ford Fiesta Issues

[little_geoff]

I just got a phone call from my friend about his fiesta, it's a petrol 2003 model.
He said his engine management has been flicking on and off, and his car has been running lumpy. He has taken it to a garage to have it looked at, they have diagnosed it and said the No3 cyclinder wasn't firing correctly, and that the coil pack needed replacing, so the coil pack has been replaced, (as this is a common fault apparently?)
Now the engine light is on and staying on, and it's still running as though it's missing one cyclinder.
(he is thinking about getting the leads replaced)
My question is this, why would the coil pack affect just one cyclinder? But also is their a simple way to diagnose the issue on this car?
I've suggested that he get himself a diagnostic cable off ebay, since he's computer savvy, so can at least diagnose it himself, but he's not that mechanically minded, so he'll end up taking it to a garage to get fixed but just don't want him to get have unnecessary work done.

Sorry it's so vague, just hoping someone could shed some light on what might be causing this issue, and also if the ford diagnostic leads are worth getting. (was looking at this item)
http://www.ebay.co.uk/itm/Ford-F-Super- ... 4179d8d424

Cheers in advance