Failed Cat?

[mojomoh]

Trying to help a mate with a V8 Disco and LPG Conversion. Passed it's MOT with ease in the UK 3 months ago, but failed it's French MOT here last week on emissions ( a bad Fail).

Before this he was fiddling with the LPG switch as it was not automatically going to LPG when started. In doing so he induced a major backfire.

Questions - first will a diagnostic check pick up a damaged Cat and second is it likely that the backfire has killed the Cat.