If they know it was bad, I say something like "Well that was a rough one, huh?" Sometimes they just need a coach to be sympathetic. If the coach is overly-positive it can sometimes come off like the dreaded pity clap.
Then when it comes time for analysis after the initial disappointment has worn off, I'll go over the splits with them and just focus on their race, not necessarily how they responded to other runners as you would for somebody who was going for points. Maybe they started out too fast, maybe they died at the end, maybe they slowed down in the middle. I try to constructively focus on that.