I wouldn’t call them rhythmless :( It is good to keep in mind Howard Gardner’s Theory of Multiple Intelligences; perhaps this individual is not a musical learner or not a kinesthetic learner. The best advice I could give is to find out what type of learner they are and apply it in a way they can understand.

And to watch how and where they carry their weight on their feet. I have a student who is relearning how to walk because they carried their weight on the sides of their feet and obtained many ankle sprains in the past.