very sad....but how does a lorry end up on it side???
High centre of gravity, especially tankers and containers, (containers simply because you have absolutely no idea of how they are packed), combined with sudden changes in speed and direction. This happens alot with tankers on roundabouts, where the driver has to turn more to avoid a collision with someone who has cut across without looking. Only takes a further quarter turn of the wheel to cause a rollover, leaving an embarrassed driver with a truck on its side for no obvious reason, the cause having long since left the scene. There is a school of thought which suggests not swerving, and having a minor bump to avoid having a much bigger one, this also keeps the cause at the scene

In this situation, and this is purely a guess towards a possible chain of events, not what actually happened because that I definitely do not know;
Something significant happens just ahead of the lorry, say traffic slows markedly, causing the driver to swerve. Or a tyre blows etc. This makes the whole thing unstable. At the same time someone travelling a bit too quickly arrives at a situation, loses control and spins into the back corner of the already unstable trailer with enough force to roll it. A split second either way and a different outcome entirely.
If you watch the demonstration clip on the Roadhawk website, you'll see just how quickly a sudden, but slight lane change can roll a truck.

Either way Guy, hope your mate gets through ok, and condolances to the families of the people in the car.