Is this what you were told by your mechanic??? Or were you at the shop when this happened???

As long as he is paying to have it repaired and he repairs it "CORRECTLY" I wouldn't stress too much.

I would like to know what your mechanics reason is for the camshaft breaking ???

Either he has left cam bolts loose or he has installed tensioner incorrectly.

As Kestas stated "It shouldn't be too difficult to diagnose" and he should be able to come up with an answer.

Keep us posted