With that said, they can try crating, RR, L-tryptophan (there are calming tabs made for dogs that could be given as chewables) pheremone spray, lavender essential oil and a few other things that can be tried short of actually drugging the dog.
What all of those things would tend to do is change the dog's state of mind and hopefully, break the association/conditioning with anxiety that has developed during travel for this particular dog.
The other side to that equation is to use a behavioral approach to condition in a new association where the dog feels safe and comfortable in the vehicle on the road.
You would start such a process with a simple game of "jump in the vehicle and get a treat, then jump out!" Make it very FUN. Yes, this will be easy as the dog likes that part, and it's fine. Make sure he gets the "game" before moving on and having him sit for a minute in the vehicle, then get a treat. Extend the time slowly, remembering that you aren't looking to change behavior at this stage, but association. So the goal is more repetitions. Keep sessions short.... five or ten minutes, once or twice a day, and give small bits of something the dog really likes.
As the time is extended and the dog remains comfortable, they can spend a little time hanging out, maybe have a cup of coffee and talk, read, feed the dog and so on.
Then... wait for it.... light up the vehicle and go back to square one with it parked and idling and do the same process again, until the dog is comfortable and relaxed sitting in the vehicle parked and idling.
If they wish to use a crate and the pup is crate trained, the crate should be worked into the exercises from the get-go. If the pup isn't crate trained, that's going to be a lot to learn all at once. Could be done but will take more time. Using a car harness and tether would be another way to enhance safety (keeps dog out of way, restrains dog in some accidents) and give the dog a more specific place to be in the truck, which is a good thing.
Once the pup is comfortable hanging out in his "spot" in the truck, parked at an idle, start moving it, but only a few feet. In, settle down, start truck, move back ten feet, treats, relax, then forward, then stop and give treats. Have a little party. Lather, rinse, repeat. Once that's all good, a little way down the block. But remember, the goal is to stop SHORT OF causing any nervous reaction. NOT to "work through it" but to erase it from the programming by making something else happen every time (dog has fun, feels relaxed) until the old programming is written over.
Then take a trip somewhere close and fun, like the park. Treats, play, etc. Home... more treats, play....
Then short but increasing trips around town.
