iPhone earpiece receiver zapping when taking a call

Hi guys

Recently i have come across a very strange issue. I have had someone bring me an iPhone that zaps them when they take a phone call. Now the first thing i can think of is that it is either a static discharge or a battery problem however i'm not really sure what part to replace? The earpiece receiver, battery, logic board or the entire phone itself ?

For the mean time i have given this person the apple technical support number, and although the iPhone is out of warranty, if it is zapping a customer i think they should replace it free of charge....surely.

Any help would be greatly appreciated!