Garmin
Forerunner 255 Series
Long battery; but wrist HR tracking is inconsistent.

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If those are your only options, get the Amazfit. The FE is pretty much a rebranded Galaxy Watch 4, which has quite crappy GPS (I should know, it was my former watch). Fitbit charge 6 doesn't have the best GPS accuracy either.
I can't comment on Garmin watches, but does he already have an iWatch or a Samsung Galaxy watch? I track all my rides with my Galaxy watch with the strava app on it. Just food for thought. When I was looking at Garmin I was lookin at the Fenix models.
I am using Samsung Galaxy Watch that syncs with my Samsung Galaxy phone
I wore my galaxy watch for 3 years climbing with no protection on it and while it got some scratches, it never really got anything on the screen and nothing cracked, it's a pretty durable piece of hardware. The only time it really got in the way of climbing was on the occasional very large sloper where you get an arm on the hold and on crack climbs. I don't wear it anymore because I no longer enjoy smart watches in general, but it was a pretty solid piece of tech in terms of durability.
Pixel Watch 4 lasts 2 to 3 days, OnePlus lasts 1 week using their default faces, Galaxy Watch also like 3 days
I've been eaten alive for saying this, but I'll say it again. Most information available comparing smart watch accuracy find them pretty close to each other. If you want something to generally track this information then Galaxy Watch is a perfectly fine pick. If you're doing more extreme sports then someone more rugged like a Garmin smart watch might be a better pick. If you want the absolute most accurate measurements then get a medical device that measures that. Smart watches are not designed or claim to medical devices. Just accurate enough to be useful overall. You may have times where it misses something or has blips but the overall result is accurate to be actionable. And Galaxy AI can offer suggestions that may be helpful in improving your health or exercise routine.
Garmin
Forerunner 255 Series
Long battery; but wrist HR tracking is inconsistent.

Garmin
Forerunner 955 Series
Best value, clear MIP screen; limited smartwatch features.

Garmin
Fenix 7 Series
Durable, multi-week battery, flashlight; high price, buggy software.
Garmin
Instinct 2 Series
Rugged, solar multi-week battery; bulky, small screen readability issues.

Garmin
Enduro 3
Multi-week solar battery, lightweight; but bulky form factor.

Ranked #1
Oura - Oura Ring Series

Ranked #1
Apple - Ultra Series

Ranked #1
Garmin - Enduro 3

Ranked #1
Garmin - Fenix 7 Series

Ranked #1
Garmin - Forerunner 255 Series

Ranked #1
Oura - Oura Ring Series