The most probable cause would be a worn out battery. I had the same issue with my iPhone 6 and it was fixed by replacing the battery. This is normal and happens to all batteries as they age. I have provided links to the battery and replacement guides: Looks like you don’t even need tools for the repair. Battery: Galaxy S5 Replacement Battery Guide: Samsung Galaxy S5のバッテリーの交換
I took out the SSD from the MacBook Air and stuck it into another MacBook from the same year. It booted onto the drive and continued to install the update. After the update was finished, I stuck it back into the MacBook Air and all was well. Im assuming its because she updated from Mountain Lion to Mojave and didn’t install the firmware updates in between. Everything works normally now!