Yeah but if you are using your HRM as a guide and not by RPE or Power, then you'll have no way of knowing any better. If the original person wants to ride above say, 150 bpm, and she's having trouble getting past 140 while at 6000 feet - what does she do? Try to get up to 150? Or does she ride at 140? How is she going to know the workout is having the desired effect if all she does is look at the HRM? Without RPE, it is just a number.
Nor would you know how much your performance is altered just by, whereby there is a fairly clear relationship between VO2 max and altitude which you cannot measure by HR.