Has my Yamaha YBR125's clutch broken?

I have been letting my brother learn to ride on my Yamaha YBR125, since upgrading myself to a CB500 - which has worked out well, except the YBR has now seemed to developed some weird clutch issue, whereby after slipping the clutch with reasonable revs for fine control, the biting point appear to shift closer to the handlebars (one has to pull the clutch lever closer to handlebars to disengage clutch after sustained period of use)... and this problem disappears after the bike is left for a few hours or less, with the biting point returning to normal (suggests overheating?. I was just wondering if anyone might know what¹s happening to my YBR? 
I think the key point is that it changes during use. It didn’t do that before. Just wondering if anyone has ever come across anything like it before? I was showing him some fine control (U-turns...etc) the way he'll be trained to do them for the test, and I was slipping the clutch like I always did in the past. He's 28 years old and has been driving since the age of 17, so he's no stranger to clutch operation.
Geoffart, MCN forums
I think that sort of behaviour is rider induced with your brother making the clutch slip more than is good for it, so that the plates overheat. If it goes back to standard biting point when it’s cooled down, there’s probably no harm done, but if the bite point keeps shifting, you’d be as well to fit a new set of friction plates. They are pretty cheap and easy to fit in 30 minutes. Don't forget to soak the plates in oil BEFORE you install them, but you don't have to drain the engine oil to do the job, just leave the bike on the sidestand and the oil should drain away from the clutch.