That is an unusually high heart rate for someone experiencing an “easy” feel to the exercise. And it is very high for someone over 70. So my first thought is not to use the stepper. I am not too fond of them anyway. If I knew this was her real HR reaction, I would not keep using the stepper and I would have her consult her doctor. It is very odd that she is not feeling the 160 bpm.
First, are you using a chest strap heart rate monitor or the hand held HR monitor on the equipment or are you taking her pulse to get heart rate?
If I planned on using the stepper, I would verify the heart rate. The handle HR monitor on the equipment (any equipment) is notorious for being inaccurate. And there is the chance of other interference with a chest strap. If someone with the same maker HR monitor is next to her, it could cause this, but it would be rare. I would do two things if I were going to use the stepper. One, take her pulse directly. And two, try her on actual stairs. Both with no one else exercising around her. If the heart rate was still over 160 on these, I would definitely get her cleared by her doctor.