A lot depends on the method you are using to calculate target or maximum heart rate and its accuracy because all formulas can do is estimate (unless it’s a true VO2 Max test) and may be significantly off for certain individuals. Hopefully you are not on any medication that increases heart rate, especially during exercise. If that’s the case follow your friend’s advice and also your doctor’s.
That all said, if you are healthy you should be free to do what feels good and the fact that your heart recovers quickly is a sign that things are working well. Just don’t ignore signs, “listen” to your body for anything out of the ordinary (e.g. light-headed/dizziness, pains, etc.) and respond appropriately. Training between 55-85% of your maximum heart rate is common and healthy for most people. Full sprints on a bike can also be beneficial.