When I was about 13 I had a friend who had lots of siblings. They all went to different private schools spread over a wideish area, but with the same pick up time.
My friend would catch the school coach and then train back to our home town, where I would then walk the 15 mins home. My friend was supposed to wait at the station for her mum to pick her up because they lived too far to walk.
A couple of times I invited my friend home to play, she would then walk back to the station for her mum to pick her up. After this had happened a few times, my friend stopped walking back to the station and her mum would come directly to my house. As a child this all seemed perfectly reasonable to me. Better than her standing outside the station in the cold/rain/snow etc for quite a long while.
However, my mum went bananas, she felt that she was being used as free child care. Often my friend's mum would be more than an hour and as hungry teenagers, we always wanted something to eat and drink when we got to my house.
Much to my great embarrasment, my mum had a huge loud row with her mother when she came to pick her daughter up from my house. The mum couldn't understand why my mum wouldn't have her daughter for an hour after school so that she could drive to the various schools to pick her other children up.
At the time I couldn't understand why my mum was so unreasonable. I didn't want my friend to be 'at risk' at the train station.
Of course now I fully understand. It was the fact the women let the arrangement develop and never bothered to ask my mum, let alone offer any financial recompense. The fact my friend's mother had deliberately chosen schools for her children and she was prepared to let her daughter wait for often over an hour at the station in all weathers. It was not my mother's responsibility to clear up the mess from another woman's poor decision making.
The reason I say this is that your friend made her own decision to have 6 children. If she can't manage the school pick ups then it is not for you to resolve her problem. If your friend is prepared to allow her daughter to sit at the school gates till 5.30 or later, then it is not your responsibility to 'save' her daughter from being at risk.
It is a hard view, but one that your friend does not seem to get.