On the TomTom runner watch (the older one), press down to bring up the menu, down to highlight sensors then right to select this. Then it will show you a new menu saying Heart press right and then you can turn it on or off.
We don't know whether he watch is the first or second model so it is possible that he could and did turn this off (but you cannot turn off cadence).
Earlier a poster saw a light flashing on the watch that means that it would be the second model, but as he has two watches it is possible that one of the watches would still be the first version and so without the heart rate data. (Assuming you cannot turn off the heart rate on the second version - I do not know if this is true)