She went through ways to destroy Astoria’s evening but couldn’t find one that wouldn’t make her look like she’s completely off the rails. Spilling something on her dress was too obvious and simply kicking Astoria out would make her look like the villain. Hermione wasn’t too sure that she could restrain herself from doing at least one of those things or preferably both if she went back. So, she just sat by the pool, soaking in her own jealousy.