iPhone 6S+ stuck at Apple logo after battery replace (needs restore).

I have a friend who's iPhone 6S+ phone had an aftermarket battery in it that's been working fine for well over a year and some change. Recently the battery started expanding. They kept using their phone, despite the expanding battery. The battery expanded to the point that it was pushing against the glass, which created a gap between the frame of the phone and the bezel of the screen assembly. The phone was used up until the point that the battery would no longer charge and the phone would no longer turn on. After replacing the battery on the phone with an original Apple battery, the phone remains at the Apple logo and won’t boot past that. I've seen on more than one occasion where a bad battery can cause a device to have to be factory reset, but it’s rare. Does anyone know why this might have happened? Regardless, I’m pretty sure the phone will have to be factory reset, but I’m trying to see if there are any other options to get the phone working before doing a factory reset. I also have reason to believe that the gap created between the screen, could have caused moisture to enter the phone.

Also… for the longest time, I’ve been trying to find PC program that’s capable of backing up a device in DFU mode, and haven’t found yet. Is it even possible to back up a device in DFU mode? I’m assuming this is not possible, because that means anyone could take any Apple device and enter it into DFU mode in an effort to retrieve/steal data from the phone.