Late chiming in; sorry about that. I wrote a reply yesterday and thought I posted it, but don't see it up, so I'll re-write. I have spent quite a bit of time in the Barrett Lakes basin, as it was one of my field sites in 2015. It looks like the SAR team covered all the ground I have to mention, but just in case another voice from the peanut gallery is useful...
Where would you have gone if your tent failed in the Barrett Lakes Basin and you needed to find shelter quickly, out of the extreme elements?
If no lightning/thunder, I'd have holed up under one of the many overhanging boulder bivvies below the largest two lakes. I saved a waypoint at a couple of them for my own reference as a good foul-weather bivy: 11s 363744 4105046 OR 363669 4105258 (+/- 100m to be safe), but I'd be shocked if the SAR team hadn't already looked here. If electricity was a threat, I'd probably run down the drainage to the woody area below the final drop to the JMT that others have mentioned (and have done this before in a mid-July storm) and waited for a break in the whiteout. If I had food and was confident I could stay warm/dry enough to survive the night, I probably would have camped at one of those places overnight and hoped for a weather window in the morning. But if he was up there fishing and was counting on calories from fish, it could be that he packed light on food, which for me might tip the scales in favor of leaving ASAP rather than holing up and waiting overnight. I certainly don't make my best decisions if I'm hungry and anxious.
Would you have tried to cross Knapsack Pass in the above-described conditions, and tried to get into Dusy Basin? If yes, why and where to?
In a whiteout at dark? Almost certainly no. My instinct would be to seek shelter and wait for a decent window, then bust out over Knapsack and Bishop as soon as I had the chance. I've crossed Knapsack in rain/hail between bouts of a thunderstorm-- not ideal (wet, slippery, but doable) but easy from a navigation standpoint. There is potential to slip and injure yourself, but it's a fairly obvious and low-angle pass... I'd wait for a window and then make a push over both passes, potentially pausing in lower Dusy at the most popular campsites by the stream/lakes closest to the trail to try to find others if I was in need of food/warmth/first aid.
Would anyone consider trying to get down to the LeConte Ranger Station, either via Barrett Creek to Deer Meadow route or over Knapsack Pass and use trail down?
In his situation, I don't think I would have gone down to the LeConte ranger station, but only because it's so late in the season that I doubt a ranger would have been there-- not worth exerting the effort to end up further in the backcountry. And those new cabins are bomb-proof-- can't really imagine breaking into one.
Not sure this is at all helpful at this point-- but just in case...