DA-they do not have internet on their phones, and some do not have it even in their homes. That is what the OP said. So, if Mattie wants folks to feed/not feed, do chores, etc. around her schedule she may have to make the effort to write it down. It is not yet her barn.
OP-your Bo needs to set some guidelines. They can be minimal, but to "best serve the clients" (that is how I would "sell it" to the BO and Mattie) it needs to be easily accessible-at a glance.
I will say, however, that I boarded briefly at a stuck up dressage barn. I am not saying anything against dressage folks, but this was a dressage barn and the folks were "above" everything-or so they thought)....and even with a board-people would get off their horses and add an hour to their ride time.....when I had come out to ride based on the schedule I had checked the day before. So guess what-I got on and rode anyway......pretty entertaining to feel their eyes going thru me as I rode my "cow pony" as the BO referred to him. (she loved him....lol) THe one guy actually told me he had trained with "anke...." and he actually had his nose in the air as he said it!
Anyway-these types, at least any I have ever experienced, will never groom their horses in the stall. They would not even consider it. THeir horses will not ground tie, and honestly-cross ties is al they know and can deal with......so many of their horses are just like described above. Rider cannot deal with horse, and only rides for lesson.
Again-I have nothing against dressage, but it does seem to me that there are more folks with this "entitled" attitude in that particular discipline than others I have been around. JMHO