In my experience, it's much easier to follow someone's tail lights on a winding dark road at night, perhaps with stone walls or high hedges on either side of the road (so perhaps not a Swiss thing so much) than it is to be in front.

It's amazing the number of impatient b******s who drive aggressively right up your tail in these conditions, flashing their lights and so on who, upon being allowed to overtake you when you find a place to pull over, end up driving much slower than you were once they have passed you and they discover they have lost the benefit of you leading the way for them.

Yeah - I had a guy do this in the snow (at night) last weekend over Ricken pass; I was doing a steady 60-70 and he was right up my arse and even flashed his lights on one bit where I slowed down because it was hard to judge the edge of the road. So I pulled over - and he did a very unsteady <60 from then on with brake lights flashing at every flurry of snow. Moron.

50 is a general limit and you can drive slower if the road requires it.

Most bikers cannot even reach 50 km/h on flat roads or uphill, so there surely is no minimum limit. This is why they are banned from the freeways, they have a minimum limit that is way above 45 km/h.

But you should not drive massively slower than the average traffic, and especially not slow down the buses that you can see on that road. That would be very antisocial and at some point it stops being prudence and starts being impeding traffic or a public service through insufficient ability to drive.
It's decent behaviour to let go of the gas for a bit if you see there are bikes and kids around or whatever.

I hope your friend then promptly surrendered her driving licence for good....