This poor girl needed a dress so I loaned her my fanciest one so that she would look out of place at the dance.
Why would you want her to look out of place?
I was in a bad mood today and rented a dress I can't really afford. Let's stay in touch after the dance.
Intentionally making someone feel out of place is pretty rude. You should do your best when loaning people close for an event to make sure they're gonna fit in. Going out of your way to make them feel out of place is most likely going to greatly upset them.
I did what I wanted. Next time I won't help someone find a dress, fine.
It isn't wrong that you helped this person, but rather that you did so in a way to make them look bad. Most people would think you were being petty and cruel for giving her a dress made to show off how out of place she is at her dance.