The three people moved the woman away, but she returned to the victim and started to stamp on her feet, BTP said.
She then poured a bottle of juice over the victim and smashed an empty glass bottle over her head, causing her to lose consciousness.
The force appealed for witnesses to call 0800 40 50 40, quoting reference 41 of 26 December, or call Crimestoppers anonymously on 0800 555 111.