You are right, she is wrong, and her friends are indeed stupid. But that's because you're the man, and gf's friends are always stupid.

You could argue that driving at 70 means you have to spend more time in the car with woman and children, thus stressing you out more. Stress is pretty dangerous when driving.

Seriously, problem I see is that most people are driving at around 80 on the motorway, if you're going slower than the traffic in the middle lane then that in itself is more dangerous than simply travelling at the same speed in convoy, you overtake and get overtaken less when you match the average speed. Conditions are critical, and those drivers around you are conditions that have to be factored in, imo. If you're sticking to 70, you're going to spend a lot of time in the slower lane with articulated lorries and fucking old people in their brown and yellow Austin Allegros, you have to go at 80 to keep out of moron lane.

I reckon next time you're all on the motorway you should go at 90 then negotiate down to 80.