The truth is your 14 yo could make your whole trip miserable without a friend. He is just at that age and your other kids are in an entirely different zone. So you made the wise decision to bring a friend. Suck up the expenses and move on. If the other parents offer activity money take it but otherwise don't ask,
My dd vacations every summer with another family who has a beach house. It is a HUGE break for the mom because the girls amuse themselves. Their family stays another week after dd leaves and both the mom and kid sat it is way less fun. Dd brings walking around money
I agree with this. You will get the benefit of a friend to help entertain your child. BTW, I'd just take 2 cars instead of doing a rental car.