Is it okay for me to ask a date's friend for her number/out while I'm at her prom?

My friend asked me to go as her date to her prom which is an entire city away, and we're entirely platonic friends - she asked some other guy first who rejected her, she ranted all this to me, and just boringly asked if I could go. I have a free schedule most of the time, so I was like whatever. But when she told me to look at a dress she wore previously, I saw the most beautiful girl ever. And I want to get to know her/go out with her sometime. I know I have responsibilities as a date, but are there any solutions where I can get to know the friend of my friend?