First, I don't know why the protagonist would even 'lend' their items to the villainess if she's been making her life miserable (unless 'lend' was sugarcoating the real term of getting a shakedown). Second, the guide only said the villainess 'disappeared', meaning her getting exiled to a place in the outskirts was something the FMC put together by herself with no evidence to show for it. Third, going back to that encounter with the Cerberus, supposedly the 'minor boss' the protagonist's crew takes on before facing the demon king, that monster was quite fearful of FMC, as if it's a pet dog not daring to bare its teeth against their owner. Why was that?
Theory: The truth about the villainess' 'disappearance' is that she somehow becomes the demon king, and that's out of anger after her engagement getting broken off and her exile. Why was this plotline not specified in the main game? Well, if the 'protagonist' really had such a shitty personality because of the writers' incompetence of making a good heroine, I wouldn't be surprised if they cut that important piece of lore either out of budget cuts in game development, or as a poor excuse to introduce some 'mystery' to the game's story when it wasn't required.