Ipad 2 still won't charge after battery, dock charger, cable replaced

I am fixing an Ipad 2 for a customer. The problem is that the Ipad would no longer charge. The customer said they tried different cables. I triple checked their troubleshooting with 3 different cables and as the customer said that wasn't the problem. Since it is a 2nd generation I figured the battery was going so I replaced it. After that it still wouldn't charge. Next thing was the dock connector. Replaced that and it still won't charge. I did notice the customer is using a 3rd party charger. I know 3rd party chargers don't regulate power as the original Apple chargers which is why you get the "This accessory may not work reliably with this device" message. I know that 3rd party chargers can fry the motherboard. But even though it is a 3rd party charger I don't get that message when plugging in. So it should be fine? So if it is not the cable, not the battery, and not the dock connector what could it be?