I'm with HP.
As a boarder, if I pay you, the BO/BM, to provide care for my horse that has been written out and then signed by myself and the managing parties, and then it is not done, its my right and obligation to be fussy and point the errors out every single time an error is made. I have worked at a barn previously, for three years, doing everything from mucking to morning feed to turning in and out horses. Its not that hard to take off a blanket or dump a bucket, especially when you're being paid to do it. Regardless of whether or not this guy was working his first day ever or his 500th day at the barn, the people who hired him and why sign his paychecks should keep him to the same standard as the rest of their employees. I would send the letter. Maybe re-write it to sound a little more casual in a "just letting you know" kind of way, but I would send it. That way, there is written documentation somewhere that at least once, he made a mistake and you brought it to these peoples attention.
If, as a boarder, I wanted to have to come in and clean up after a barns employees, regular or not, I'd save myself the monthly expense and keep the horse in my backyard.