The fears of your customer is slightly far fetched, the phone will always heat up when in use but not because of the battery, the chance that even a broken battery are low as long as you don't puncture it and aslong as you don't have a defective part the chance is impossibly low. If it does come from a reliable source (such as iFixit) they continously check their parts to make sure they are up to quality standard. And if they are concerned about that then an apple genuine battery will get warm when charging, its just because of the flow of electricity it will get warm.