According to the step-and-repeat behind Reese here, this event was partially sponsored by Roberto Cavalli. I wonder if they sent her something and her stylist had to write back and be like, "you guys, come on. You know you are not on brand for Reese," and their PR people were like, "yeah, yeah, we know. It was worth a shot?" and then everyone planned when they were going to get cocktails.
This is what she ended up wearing, regardless:
I'm kind of into it, although it does remind me of an incredibly formal screen door.