It's a good guess. It does happen. It could be the case.
But in this case, since he says it's better if not gone after warming up, I think it may be a fluid issue.
Cracked fork wouldn't work better warm. Air bubbles in Clutch slave wouldn't go away after driving. Unless their failure was so slight, that normal drag from cold gear oil would be too much and push it over the edge of not shifting (input shaft still spinning too fast, even with clutch in)
definately should inspect the clutch fork, and slave, just to rule things out.