Just to repeat what has been stated before: you need to have a physician’s approaval for working with this client.
The heart rate response is so altered by beta blockers that you can only use the RPE scale. If you use a heart rate monitor, you would find that the heart rate seems to hit a ceiling even when the client tries to work harder.
People on beta blockers often find that they feel tired and have low energy. You need to work around this client’s comfort zone.