Cambelt failure, advice needed

[ash951]

Hello,
My waterpump failed and the pulley sheared off, this has caused the cambelt to shred, and the engine obviously cut out. A piece of metal has chewed up the crank gear so I need to replace that.
I had a mechanic look at it on site and he has said that I will need to replace the cam pulley as there is an inertia device in there that will allow the camshaft to freewheel independently from the cam pulley if resistance is met. This is news to me, is this the case?

Thanks in advance,
Ash