we both wore a watch and my HR never went above 125 and averaged about 115-120 at a very slow jog; about 8 minutes per km pace. her HR was around 170 after every 800m or so, at which point we had to walk for a while. she is not fat, her respiratory system must just be really weak. but now i understand why people at the park don't run faster, it's because they can't sustain even a relatively slow pace for more than a few 100 metres before they get into HR zone 5.
also her HR took a long time to come back down when we started walking each time. when my HR was at 85-90 or so when we were walking, hers was still 150 after she had been walking for 3-4 minutes. i had a heart test at a hospital once and the doctor asked if i was a marathon runner because he said my heartrate came down very quickly at the end of the exercise stress test. so now i realise what he meant.