She got a bit unlucky tho I must say. I've lived there for 10 years, and have never seen it happen. I can only think of two friends who've been mugged during that time as well. Assuming 30 friends that's 150 man years per mugging :)
The touristy areas thing is probably a big deal. I've also spent ~10 years in London, some of it in Zone 1 without witnessing any attempted street thefts or knowing of any friends being mugged. Approx 1 week in Athens: two, one targeting me (neither successful)
There might be a difference in street crime between London and Athens but it's not that big.
20 years here (in this stint) and I've never seen it but I do tend to avoid crowded places (social anxiety) except when peak travelling.
I do know people who've been moped'd and mugged - never suffered either myself (although I apparently have a "resting hate scowl" according to my sister which scares people away.)
My wife had her phone robbed from her in this way in Canonbury (N5) 2 years ago.
It was very scary for her since they went up onto the curb and pushed her onto the tarmack as they grabbed it. It happened at around 6am in the morning at most 20 metres from our flat which was on a quiet terrace of victorian houses.
It's not just touristy areas. It's probably easier for the motorbike thugs to operate in places which have relatively quiet roads that they can escape on. I suspect that if you live in a nice area, but close to some of the inner city ghettos you are most at risk. I've also seen it happen in and around the parks.
I think it depends on the area - I've seen it happen twice in broad daylight on Clerkenwell Road near Farringdon station over the last couple of years and it's happened to a friend of mine near Waterloo.
I've been living in central London (zone 1) for 3 years now and never had any issue (nor my partner, nor anybody I know).
It might also be due to the area being touristy or not.
I remember a crime map (I can't find out this link anymore...) showing that the side of London bridge which faces Tower Bridge had a high crime rate whereas the other side was absolutely safe. So if you want to cross London Bridge without taking a picture of Tower Bridge, pick the correct side :)
London is a big city though. As most big cities, I would never expect it to be totally safe.
Do you live in central London? It's hard for me to imagine you wouldn't come across it if you lived within Zone 2, say - I only lived in London a relatively short time, but some guys tried to mug me (I didn't have my phone out), I found a phone on the street and returned it to its owner, and I lost count of the friends it happened to, the incidents on the road I lived on, etc.
I have lived in central London for 25 years and never experienced this, nor do I know anyone who has.
However, I know the problem exists and I have just been lucky. I am not claiming that my experience invalidates that of others - just adding my anecdote to the pile.
I think it partly comes down to what you look like and how you act on the street. Of the people I know who have had their phones snatched the common denominator was usually being too comfortable standing on a street staring at their phones. It's hard to miss kids on bikes/mopeds if you're watching for them, but if you're not and they appear then they'll go for you.
Depends how used to living in a big city you are. I think those of who live here have a bit more situational awareness (that said, the moped thing is new and maybe they roll up so fast situational awareness won't help).
I've seen it happen once & heard of it happening to a couple of friends of friends. Probably depends on the area - the one I saw was on City Rd which is known to be bad for it.
EDIT: she