His tech broke it. If you are rushing a valve cover removal, that part can get hit by the valve cover. That plastic part is sealed inside the valve cover so there is no way for it to end up in the belly pan unless the valve cover area is opened up. The plastic is slightly brittle with age but any good mechanic will know that and be careful.
That part does not go bad. There are plenty of 200k mile I6 BMWs on their original timing chain hardware. It does not wear out on a properly maintained engine.
A complete new timing chain/guide is not required. The broken part is $45. He will need to remove the sprocket to be able to remove and replace. Special tools are needed to hold the camshafts in time. If he does not have these tools, he is not a BMW specialist.
https://www.ecstuning.com/b-genuine-...l/11317592850/
Here is amateur that broke theirs doing a vc gasket.
https://www.e90post.com/forums/showthread.php?t=1566915