If anyone out there doesn't temporarily have time for their horse, rest assured it's OK for the horse to have a break from work once in awhile.
Sometimes the thinking is, "maybe I can get someone else to ride my horse so that he can get some exercise". That is not always best. When you let your horse sit around without exercise, when you go back to riding, that horse won't have learned all kinds of new bad habits. It will have the old ones, it will probably be more lazy and easily distracted, but it won't be messed up from someone else riding it.
There is of course the exception when the person riding the horse is a good rider, or the inexperienced rider is in a lesson situation. Then the horse will be getting some good training. Any horse can go bad no matter the age or attitude of the horse. It all depends on the rider. When it is your horse you will be doing your best to make the horse his best, this may or may not be true of someone else who does not own the horse.
A horse is a mirror of it's rider, don't think you are doing the horse a favor by allowing someone without enough experience ride your horse unsupervised. The horse you get back will not be the one the one you loaned out for exercise. So in a nutshell, no work is better than bad work. Just turn him out and let him be a horse for awhile.