Sounds like she wanted a guaranteed "date" so she didn't have to be dateless, even though she isn't actually attending in a date-like way. If I were your DS I'd say "since you've made other plans, I have too. See you at the dance." At that age, teens who are truly dating or the new pre-dating lingo of "talking" often end up broken up or changing who they are "talking" to between the HC ask and the HC dance, so him saying "I've made other plans" wouldn't be an outlier and really just mirrors what she's saying to him.
Mom to two amazing DDs ('07 & '09) and a fur baby.
Gluten free since Nov '11 after non-celiac gluten sensitive diagnosis. Have had great improvement or total elimination of: migraines, bloating/distention, heartburn, cystic acne, canker sores, bleeding gums, eczema on elbows, dry skin and scalp, muscle cramps, PMS, hair loss, heart palpitations, fatigue. I'm amazed.