Just out of curiosity, is she using the same tool to measure her heart rate? IE, is she wearing a heart rate monitor on her own body, or is she using the heart rate monitors on each piece of equipment? That might also contribute to the variability.
Heart rate is a useful indicator. That said, it can vary day to day, based on hydration, sleep, medications, stress, etc. So I like to use RPE as well. Does your client have good body sense in general? If your client is feeling like she is working “easy” on a piece of equipment, she’s probably right!