The story still doesn't play out as it would in real life. The only instance I ever saw where people were immediately fired and escorted out by security was if they were accused of leaking information, and even in that instance their materials were collected by the security guard placed in a box which was then delivered to them as they waited in a conference room. I've seen quite a few people fired on the spot for stupidity but not in the manner you describe.

She would definitely be given time to clear out her desk and take her things, and she would definitely have time to at least give a quick indication that she is leaving to her coworkers as she was clearing her desk. Deleting from the system rarely happens within 5 minutes, and I'm not sure how you or her would know that as you claim she didn't even get to go back to her desk.

horrendous work conditions in the city cause absolutely zero impact in Oxbridge

Not true at all, everyone expects the work to be hell, but every firm is expected to have a very complete support system in place especially for entry level positions, failure to provide that will have repercussions on recruitment and firms are aware of that, if they acted shitily you can be certain they'll pay whoever off in exchange for NDA. Some of the very best people I've worked with have all had rough patches where for whatever reason you lose a step on the rest, nobody is immediately fired over that. I went to a school that supplied many individuals to the largest firms in New York every year and everyone on campus was always aware of stories surfacing about unusual treatment, I can't see Oxbridge being any different.