Sometimes the 5th korotkoff sound (the noise they listen for to determine the diastolic reading) can be absent, therefore giving a zero reading. though this is more common in children and pregnant women. Although it is present in those with high cardiac output
, which is probably why he was referred. Usually practitioners will do the reading from the 4th korotkoff sound, giving a value of 60-70 as norm. After just re-reading your question, I hope they weren't just relying on a machine and were checking manually as well!