Under these delicate circumstances, I would say:
1. I don't have liability to cover anyone but myself or my husband (as the Owners) to ride the horse.
2. Horses have varied personalities and levels of schooling. She is not your ordinary backyard/pasture pet that just anybody can ride.
3. She recently had a baby, sometimes has Post-Partem Syndrome, and will most likely remain 1,500 pounds worth of hormonal until her system gets back to normal from nursing a baby.
4. She is essentially a college graduate with a _______ degree in Dressage (what would 4th level dressage equal in a human degree?) That means she is extremely sensitive to physical signals; inadvertently giving her the wrong signal at the wrong time could end up with you "falling" off.
4.1 Annnnd that goes back to number one regarding no liability insurance
I have found more-often-than-not, if you can translate horse talk into language non-horse people understand, they have a more clear understanding, and hopefully fear, of why they don't need to get on your horse
I have been riding and training trail horses 53 of my 65 years and I
wouldn't ask to ride your highly schooled horse for fear of giving it the wrong signals. There's a big difference between riding a trail horse and riding 4th level Dressage
And for that fact, she wouldn't be riding my trail horses either. In this day and age of law suits, I would simply say "nupe, you can't ride, I'm not watching you get dumped and land in the ER at my expense
I hope this helps