Ah, so she is from New Addington. Frankly, there are areas of London where this shit happens all the time, and New Addington is one of them.
I agree that there are problem MH or substance misuse issues going on here, though. That poor kid. And they were both lucky that she didn't get slapped.
My mum once intervened, at midnight in Brixton tube station, to try to shut up an out-of-her-head young girl who was screaming racist abuse. Sadly, passers-by assumed DM was part of the problem, and soaked her in beer.