Thank you for all the useful replies.
I consider myself an experienced, safe, and sensible hauler. (I also taught my twins sons to drive at the same time, so I have nerves of steel!) I was asked directly for advice. I suggested a step by step approach as someone described above - go to the high school parking lot, then back roads, then the highway, all empty. Then load the horse and go around the block etc etc. Well, all that was for naught - first trip was with horse, on a freeway. There were loading problems. Yikes!
I knew this was going to happen, but hey, I tried. I even made bets with my hubs that all my suggestions would not be taken seriously. Thankfully everyone made the round trip safe and sound. I did have another talk with the new trailer owner today and tried to make my case again, after gasping in horror! I may have at least gotten an admission that there needs to be some serious loading practice before the next trip. The horse is probably a great loader, but you know that sensor they have that picks up everyone’s urgency to leave??? The one that connects to the “hell no” button?? LOL! I think he’s got her number, honestly.
You know the saying “Good judgment, that comes from experience. And experience comes from bad judgment!” I’m cool with that except when there’s another living being that you allegedly love a lot involved. Try to at least stack the odds in your favor, eh?