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.

You also need to factor in frequency of tube services and looking at the LA peeps, you need to consider the percentage LAX-LHR passengers who are from LA and coming into the capital. Bear in mind the regression of passengers who are simply using that flight for convenience but then travelling on to other parts of the country or not leaving the airport at all be transfering onto other extranational flights.

Consider then the percentage of tube passengers who actually use LB and cross that against LAers in London altered for tourist density and tourist transport modes. You'd be lucky if the number of LAers exiting LB exceedes more than 100 identifiable individuals a day.

The really awkward factors that will be hard to determine will those of two people with a clear and direct connection who are within line of sight of each other actually noticing each other.