Many years ago, we were approaching the top of the Bishop Pass as a day hike. A guy heading down looked at us and said "Those are light packs." We assured him that we were returning to South Lake as soon as we tagged the pass. He looked at his watch and said "OK, you have enough time to get back." I appreciated his concern and thanked him.
We were once returning from Cloud's Rest and were maybe 2 miles from the trailhead when we saw a couple heading up. It was maybe 4PM. We asked if they had a flashlight. They did not. We told them how long the hike took us and that they might not be back before dark. They said that they would pick up the pace. I don't know what happened after that.
The situation where someone plainly looks bad with some evidence of mental confusion is a difficult one. You might need to take them by the arm and lead them down the trail. However, it's public land. You can't wrestle someone to the trailhead who is determined to push on. Maybe the best you can do here is to contact the rangers or SAR as soon as possible and report a disoriented hiker who refused to abandon their hike.
As far as breaking off your hike to help someone else goes, I have no problem with that. The trail will always be there. I don't want to spend the rest of my life thinking that someone died because I was too selfish to help.
