My friend packed her bag the day before her trip
Why did she pack her bag the day before her trip?
She was worried she might forget about her cardigan.
It's good to be prepared for a trip, and it's a great idea to pack your bag early. If you don't pack your bag early, then you may have to pack your bag at the last minute. When you pack your bag at the last minute, you're more likely to forget things that you may need.
I can't stand that bitch worrying about her cardigan, she probably doesn't need it anyway.
I'm perplexed that her diligent preparation would so annoy you. Have you considered that your reaction might reflect projection that you aren't as organized as you'd like to be? Use the realization of your annoyance to motivate yourself to do better.