scirocco,

there are a lot of possible causes of improper idling ... it would be best to bring your friend's car to a honda casa ... insisting on non-honda mechanic could result in even more damage to the car ... modern engines, being computer-controlled, are now too complicated to troubleshoot even for a seemingly simple idling problem ... lots of sensors and actuators that most talyer mechanics are not aware of

note that when the problem has been diagnosed properly, for idling problem, the fix may be as simple as cleaning some parts on the throttle/intake manifold assembly or injector ... when i had an idling problem few years back, the Electric Air Control Valve (EACV) was found dirty and only needed cleaning ... the EACV have a significant role in maintaining proper idling ... a loose contact at the fuel injector terminals may also cause erratic idling

so please tell your friend to go to a honda casa if he wants the problem diagnosed and fixed properly ... but be aware that even honda mechanics can err or too lazy in diagnosing a problem and may suggest replacing parts unnecessarily, in which case, you may have to find another honda casa with a more competent mechanic ... i've only twice experienced idling problem on my honda esi, not a single part was replaced

hth