This info is available in-game, but understandably it might be hard to find since I probably haven't communicated it well within the game itself. While playing, at the bottom there is a 'Help' button. In there, you can find a bunch of information on each predator.
Harpy requires having an upset stomach. You need Amelia to struggle while unwilling. If Amelia is willing, then her struggles won't upset the Harpy's stomach.
If all else fails, you can utilize Simple Load as a means to escape as well. This will put Amelia outside and back on the diary space.