Allroad 2.5 tdi camshaft broken at 64,000 miles - why?

[Bcoxy]

My lovely Audi did the unthinkable when we were in France and let us down big time – in the middle of nowhere it made a horrible noise then ground to a halt and made further horrible noises when I tried to re-start it. The helpful French mechanic who came to our rescue (courtesy of RAC Europe) pulled a face and told me that the engine was kaput. I told him I’d recently had the cam belt changed and did he think it could have snapped? He thought it possible. Anyway one sorry journey back home via tow trucks and taxi and the car is now in the garage where we had the cam belt done. I’ve been round there and he’s got the whole front off and engine covers open and he’s shown me that the cam belts are fine and not snapped. However the cam shaft on one side is! What the hell could cause that? I’ve never heard of that happening on any car, never mind an Audi that’s only done 64,000 miles. Any ideas? Could he have mucked something up? He did show me that the cogged gears at the end of the cam shafts (there are 2 next to each other on each side) have broken teeth, which look to have been broken for some time, to judge by the discolouration (but then I’m no judge). Any thoughts please, anyone?