She knows the Heimlich she won't choke, not Daphne (farsick) wrote in caged,
Daphne Greengrass
Daphne decided to come dressed as a water nymph/naiad. Her costume consisted mostly of stuff she or Astoria had around and altered together. A dress like this (probably without that weird face - just hazarding a guess here), accessories of the aquatic/botanical/naturey sort, and hair and makeup courtesy of Astoria's mad skills. They even managed to find a nautical-type scented perfume. All in all, she's pretty pleased and comfortable, because you need to be comfortable to bust a move, yes?
The ironic appropriateness of her costume will likely dawn on her later, when/if she returns to find her Common Room flooded. Womp womp. Rock on, party people.