Bllw, I am very sorry to hear that your friend is missing. I have been praying for him since seeing this news.
I hiked the standard route with a friend from 11:50pm Wednesday night (6/12) until we returned to Whitney Portal at 8:05pm (Thursday night 6/13). We sumitted at 12:05 pm on 6/13. We did not notice anything out of the ordinary besides tired hikers on the way down early Thursday morning. I am fairly certain we did not pass your friend. There were several places an accident could happen and from the 6/13 perspective, this is where I would personally focus search efforts.
1) Whitney Summit to Trail Crest - Particularly around the Needles and Mt. Muir traverse sections. There were some difficult sections on the traverse where footholds were limited and holding on to rocks was required to safely navigate the trail. Some sections did not even have a full foot width of melting ice ledge to stand on. There were steep drops that would cause injury or be hard to ascend from if someone slipped.
2) Between Mirror Lake and Outpost Camp. This was a difficult section to navigate as the trail was largely covered with snow and boot trails went every direction. While going down the mountain we saw many boot trails heading straight for Outpost Camp but of course between us and Camp was a waterfall and a steep/nearly cliff area. The actual trail goes to the left of Outpost Camp (from the perspective of descending the mountain), swtichbacks, and comes around to the Outpost Camp at a much easier angle. Another hiker recently posted about falling in this section with his dog.
3) Shortly before and after lone pine lake we found the trail hard to locate on the descent and had to rely on GPS several times. While the general trajectory one would have to go would be towards Whitney Portal there were places here one could get lost and even hurt (especially if tired).
4)Depending on what your friend was using to find the trail, he may have, like us, utilized the summer trail route via GPS. This route does not get as close to Consultation Lake as the snow/winter route, but instead climbs a ridge to the right (ascending) or left (descending) of Consultation Lake. While ascending, the trail traverses towards the Chute and Switchbacks just shy of Trail Camp. On most maps, you can see this ridge climb just below Consultation Lake because the trail actually turns 180 degrees on the ridge ascent before coming back around at another 180 degree turn.
On the ascent we took this route and had to cross a very steep snow field in excess of 40 degrees (having climbed Avalanche Gulch on Shasta, I know 40 degrees but I could not advise how much steeper this area was). We kicked steps into the ridge to cross safely. Most people did not take this route and a fall far down the ridge could go unnoticed for some time.
5) On our return trip, the 2nd water crossing from Whitney Portal (ascending) or 1st from the perspective of descending was a two fold waterfall, filling the trail with water for 30 feet and than plunging over the trail into a steep waterfall below with rocks and other debris. We took two paths through this - I walked through the water up to my knees and the current was not bad so I stayed near inside of trail and away from danger (though I got plenty wet). Others crossed on semi-submerged rocks near the edge. A slip from the rocks would have been bad. This crossing was not nearly as bad at 1:00am so I do not know what it looked like if your friend made it here late day on Wednesday. For this reason I am ranking it the 5th location.
Will try to get a full trip report done in the coming days but wanted to send this to you as an idea of conditions on the trail (from one hiker's perspective) within 24 hours of your friend's hike. Perhaps other hikers from Wed. and Thurs. may have additional or alternative perspectives of risk areas that may assist in the search.
