With all due respect to your builder, (but not much) ......... he's talking tosh.
One of the current Building Regulations specifies mechanical fixing to ridge tiles (screws and brackets) rather than relying on mortar alone. Sometimes, a ventilated ridge is needed.
Whether you have a ventilated or a simple "Dry Ridge" system, it doesn't alter the fact that is should be watertight.
Whichever it is, it simply is not installed properly. We all make mistakes. The definition of a good tradesman is one that will gladly rectify them.
Press the guarantee company to do the right thing.