Since he is stock, unrooted, and non-swappable battery, he may have a faulty battery!
Every battery has a built-in circuit that reports its charge, temperature, and stability. That's why every smartphone battery has 3 or 4 prongs (contact points), rather than just the 2 for positive and negative. Best case scenario his percentage is just reporting incorrectly, worst case he may have an unstable cell that could combust. I would recommend looking up disassembly videos for the phone, and replacing the internal battery if it is accessible.