What are the odds?

ok, serious answer:

I would look at it like this:

1) On any given day, what % of folks in London are from LA?
2) How many folks use London Bridge (LB) tube per day?

From this, you get the number of LA peeps who use LB per day:
Flights per day LAX-LHR: 10
Seats per plane: 350
Avg occupancy of plane: 70%
Daily new arrivals from LA: 2,450
Avg stay in London: 14 days
# LA-ers in London per day: 34,300
London population: 7.6m
% London pop who are from LA: 0.45%

LB passengers per day: 187,500 (extrapolated from 07 & 08 LB exit data)

So assuming a tourist from LA is as likely as a Londoner to use LB station, you should have 851 LA-ers using London bridge per day.

The next step is to ask what is the probability that Aram's friend is one of those 851, and what is the probability that Aram is one of those 186,649? Moreover, what is the probability of them being there at the same time?

Might pick this up later, bit busy now. Someone else can run with it if they want to.

If we're going to do it, lets do it properly. We need passenger figures for each station to work out exact probabilities of a person at any one time, in London, using London Bridge station. Elguapo's personal probability of using that station can't be worked out without further input. Why was he there? How regularly does he use that station. If it was a one off due to another reason then the probability gets more complex. More details please.