I doesn't hurt to call and have the authorities do a welfare check. If you can go talk to the owners in person and offer advice/help then do so, but if not a call hurts nobody, and can only help the horse if abuse/neglect is found.
I bolded the part you must've skipped over... No worries.
I don't know about OK, but authorities here are required by law to visit a property if there is ANY chance an animal is being mistreated, abused, neglected, lack of necessities provided, etc. If it turns out to be unfounded, GREAT! But if it IS bad, or on the way to becoming a bad situation, they can stop it or bring it to an owners attention before
it gets worse. If OP is unable to educate the owner in question the next best thing is to call, if nothing else they can have a rescue get in contact with owner about proper care/husbandry.
I would be not be pissed off that someone would care so much to stop or call if one of my animals were in clear pain or danger... Nothing to hide here.
There are too many people in this world who just look the other way and keep on a drivin'.