Why do these dogs have constant diarrhea?how can i stop it?

so i am pet sitting 2 sheepdogs. i know one of the dogs has had diarrhea for at least two weeks. each time they poop, its nasty! Both of them have it and i know the owners know about it, so is there any natural solutions? and i know this is not normal, but they otherwise are in great health. they eat and drink plenty. they get exercise sleep.