I hear you woot and I am the step mom to a 16 year old so I know what you are saying. Maybe things have changed significantly since my days in college (I'm 40 so take this for it's worth). My best friends and I shared everything...clothes, money, food.
I don't see inviting or letting someone who is just acquaintance come on a trip with you, especially one who is a bit younger and could hold you back conceivably from doing what you want on Spring Break. i'm not sure they thought she would hold them back.. they were all underage and prob had fake ids. and I think she traveled in a a 'fast' crowd IYKWIM and could hold her own And she was staying in their room...makes her part of the group even if she wasn't really a friend. I also know never in a million years would I have let an "acquaintance" borrow my clothes. i hear ya but it would not suprise me if she borrowed them w/out asking and thats why the other girl got po'd and demanded them back..both of her parents said she was used to getting her own way That privilege was only for my best of friends. I just feel like they had more of a connection than any of them really want to admit. And maybe they are just downplaying it because BD is gone and they want to remove themselves from the whole thing.
Not putting you down or questioning you since you are local...you have more insight than I do.
Just saying it still doesn't sit right with me.