If anything, the jostling will help keep the yeast in suspension.
The one thing I'd say is that you do want to keep carbonating bottles in a bit of a warm environment in general (i.e. 70-75 degrees). If your brother will be keeping those in the cabin of the vehicle and keeping the A/C running cold, just make sure he gets them into a warm area of his house when he gets back home. 9 hours at 65 degrees won't be a problem, but you definitely would prefer them to be a slight bit warmer for carbonation.