How can I stop my cav chewing slugs Training

It would be great to know this info might save other little pups.
I have a friend nearby who's pup died and although she didn't have an autopsy as he died at home she said he liked to eat slugs also. The worrying thing is that she had mentioned this to the vet and they just said 'no wonder he has a dicky tummy'

I guess there are good vets and bad vets like every other profession and we have to hope we get a good one. George saw three vets so I think he had really bad luck:-(