In what way did the timing belt fail? did it snap or become loose..did the belt shows signs of any damage or excess wear on any edges etc?
How did they check for a worn camshaft?
On the invoice they should of replaced the timing belt along with all the associated pulleys/water pump? and if so what brand did they install?
Ok in my opinion generally premature reason for failure can be incorrect fitting..In most cases a cam belt breaks because its maybe too slack not properly tensioned.. the water pump collapses, a guide pulley has seized, the tensioner has failed or all 3 have been substandard components aswell as the belt itself or foreign matter or something has caught up in the belt and cause it to snap..missalignment of the belt not running true can also be factor..I cant really see camshaft wear cause a belt a to snap unless the cam locked up for what ever reason...Nornally as above is the most common causes for premature belt failure