It's not that hard to ride hard for awhile, then simply take one's own pulse for 15 seconds then x4 to see if the HR monitor is in the ballpark, in which case it is very likely reading accurately. There's no particular reason to believe the HRM is 50 beats off. He wasn't going all out. No reason to believe that max pulse rate for a 63+ year old with resting pulse rate in 40s is going to be 185+. Max heart rate does drop with age, even though it will often depart from the usual formulas, which is why field test and zones based on LT instead is recommended. Last Saturday I rode a 14 minute (for me) hill climb near all out, I am 40, I hit 168 max. 19 year old kid on same hill (faster) said he was 190+ pretty much the whole way. People's ranges differ, and older is expected to be lower.

I have same Cateye strap & computer. First strap I bought started going flaky after working fine a few months, it was replaced free under warranty. But it didn't malfunction by reading 50 too low. Things it did were:
- stay frozen at 78 despite my heart rate going much faster
- then some drops to zero, not reading at all
- with a changed fresh battery, climbing to 199 and staying stuck there
Replacement has worked fine for months.