I've been there it was terrible!!

My boys never did it unless the timing wasn't right for feeds but DD did it not matter what i tried. She suffered from silent reflux and i did read that the positioning of the carseat can play havoc with the issue. I wonder if this could be an issue with your friends little one?? No amount of pulling over, feeding or soothing could change the fact that the second she was in her seat (and not even driving again) the screaming would recommence.

Even now we are yet to turn her around to front facing so that was never the issue for her and she always had her brothers in the car and could see them quite clearly. Me personally with my daughter i believe it was the reflux.

For us it lasted for the most part of 5-6 months. We went away at easter time (when she was 5 months) and we were so worried about the drive but we managed, it wasn't easy though.

Thankfully this second part of the year she's much better and sleeps and plays happily in her seat.

It can be so distressing for anyone in the car. I'm really not too sure if there is anything you can do if it's a reflux issue, but the only other things i can think of would be is she hungry, tired, too hot... all those sorts of things.