How about a slightly different approach to ensemble playing? I'm thinking teach your pupil to busk 2 chords (perhaps just power chords or even root notes to keep it as simple as humanly possible) on ukulele or guitar - strumming only, on the beat, but keeping in strict time whilst you play the melody.
It may help change the approach, allowing the pupil to understand what continuity feels like and sounds like, without all the added busy-ness of working out notes and rhythms at the same time.
Of course you could do this exercise on keys, but it may not be 'different' enough to feel novel and thus memorable. Plus I believe the physicality of playing an instrument with larger arm movements can help with feeling pulse. You could transfer it on to keys on a later occasion, once the basics are in place.