YANBU at being upset at being taken home against your will.
But in fact, your will wasn’t very strong at all was it OP.
I’m wondering if you are so very passive, that when the vomiting started, she felt obliged to take charge and get you all home….to put an end to the nightmare situation. At least the non-vomiting children could sleep in the car. Maybe after 4 days in wet and windy Wales, your friend had simply had more than enough anyway and the vomiting was the last straw for her. Who knows.
Or maybe you didn’t want to be left alone on holiday with your son with a poor weather forecast and no company/nobody to help make decisions or entertain and discipline your child?
Who knows exactly what this situation looked like to your friend.
It’s quite difficult to empathise with the scenario which you say you really didn’t want but did absolutely nothing to object to.
Surely you don’t allow yourself to be hijacked in the middle of the night in your own car, with a vomiting driver and four vomiting children, all against your will, without putting up a fight ?
Sounds to me as if the friendship might be over anyway OP, so no need to worry about that. I hope you and your little one don’t get sick and I hope you are all home safely by now.