Just going to lay my two sense on this. She was probably using a crappy ass third party charging cable, and the case she was using. Yes the iPhone 7 gets hot but with a case like that, facing down so no air circulation from the room can get to it, I wouldn't be surprised if that's the reason why it melt. Bad ass case.
She also said "The past two nights it hasn't been on my bed at all," Helps explain the issue, phone charging on your bed. You fall asleep and now its under your pillow with a bunch of apps still running.