Non responsive, no boot no itunes

Hi Guys,

I run a repair store focusing mainly on Apple products. In the last few weeks I have encountered 5+ Iphone 6/6+ and 6s models all with the same problem. The problem is the Iphone is totally unresponsive, no charge being drawn when on charge, no itunes and no life even after several LCD + battery changes. I was told by one of the customers that they had had the same problem previously and a repair store had fixed it. They only charged $25 and said no to let the phone ever 'run flat'. The customer said it worked flawlessly for another few weeks but eventually she ran it flat and it would not boot. I have heard this before but dont understand what is done to get the phone to boot. The customer desperately wants her data off the phone and cant understand why I can do the 'fix' the other repair store did (other store is over 2000km away!).

This problem has occurred to both dropped and non dropped iphones.

Cheers.