I do keep an eye on my HRV - I have an Apple Watch and the paid HearthWatch app.
My resting HR is below 40 and I have been woken up while in the hospital, as it may drop close to 30 when I’m in deep sleep.
Apparently they fear you can go into cardiac arrest, but once you tell them this is normal, as you’re an active cyclist, they stop worrying 
I was hospitalised three times after being hit by a car, which probably is one reason why I’m not a big fan of our ‘safe cycling network’. The effects of the first two have added a lot to the long term effects of the car crash I had in 2002, when a van hit me from behind.
I started indoor cycling/spinning after I came out of rehab 1.5 years later. I used to be very active in all sorts of sports, but my ‘career’ got in the way and after my car crash I decided to start as an independent contractor.
In the winter, my load normally drops, as I do not get out much and train indoors. I used to do more and longer sessions, but I try to avoid anything over 2 hours. I may occasionally do a longer session, but I usually do two sessions of 1-1.5 hours.
However, in the winter of 2018-2019, I was out for 3 months because of a triple hernia. That has been a problem ever since 2002 and it’s still there, but I get shots every 3 - 6 months.
Not sure how long that will continue to be effective, but so far, it’s helping, although getting them is excruciating and the benefit may last shorter or longer, you never can tell.
Maybe one reason why I do not ‘suffer’ a lot and recover more quickly, is because I’ve adapted to my usual SS training and my HR is relatively low. Perhaps your friend can shed some light on that.