I see no problem with her choice. Prom was on our last day of school my senior year.....my buddy and I skipped out early, did some burnouts on my Chevelle, and then went Up North. No regrets. I'm pretty confident my memories from our trip are better than any I would've had at prom.
Also, I bought my mom's XJ off of her early in my senior year and my parents said as a graduation present they'd either throw me a big grad party, or buy me a lift, wheels, and tires for the XJ. I chose the lift, and while I got a TON of use out of it, it's been a slippery slope from there. Of course Trish is already sliding down that same slope so let her do what she wants to do. Or as a grad present to her, buy her the locker and send her to prom!