There is a general rule among horse owners that you do not ask someone if you can ride their horse. It is considered rude. A horse is a very personal pet. A loved pet is not let out for others to ride. If you are non-horse owner and you wish to ride a friend’s horse then you must simply wait until the owner offers to let you ride – NEVER ask!
As a horse owner I have been asked a few times, but not many and when I was asked I simply ignored it. Usually those people got the hint and never asked again. If they want to ride a horse then they can go to a commercial trail riding outfit or take a riding lesson.