Hmmm...maybe it's just me, but Harley wasn't actually doing anything wrong when Robin and real-Batgirl attacked her. Admittedly, dressing in Barbara's costume was arguably tasteless and provocative. (being an IRL troll, DCU style) but does that actually justify physically attacking her? Dressing up as Batgirl isn't actually a crime.
I'd love to see the scene if they took her to the police station for this. "Uuum I hate to tell you this, but fancy dress isn't illegal guys"