Beta Blockers aren’t effective for heart rate zone training because beta blockers slow down heart rate.
A better method for anyone who uses beta blockers is the Ratings of Perceived exertion (RPE) scale.
The article below from ACE has a nice explanation of RPE. Two scales are shown because the descriptions on the 1-10 scale are supposed to be easier to understand.
NOTE Doctors usually prescribe beta blockers to people who have hypertension or a heart condition (heart attack, abnormal heart beat).
You probably won’t work this person up to the highest level; check with the client’s doctor to determine the appropriate level.
Hope the helps you out!