I've heard of only a couple of incidents like this. Usually, the cause in earlier YZF's is camshafts partially, or momentarily, seizing in the head. I'd pull the cams and have a look at the journals and saddles, and also check the clearance.
I'm not aware of any issues with stretching chains, or of a tensioner weakness. The tensioner was suspected as a cause of the valve failures in the '06 250F, but the speculators turned out to be wrong about that. YZF tensioners are one of those things that are so simple that they're brilliant. Basically too simple not to work.
Thanks for the input grayracer. I did ask him that. I know that the cam caps are easy to overtighten. He says that he didn't pull them but maybe there is an issue there anyway. I will make sure that he pulls them though. Good thought.
I have never heard of the tensioner going bad on the YZF's but I have seen it happen on the big Kawi's and Zukes. I wasn't sure if something changed on the newere bikes.