It is not uncommon for people to target dealers after shows. I know a few other dealers who have made the same mistake thinking they are safe, but in reality not, and people broke into their car and stole their stock. I agree it was not a good move on his part, as an exhibitor I never leave my material in the car alone. Even though it is covered, trying to replace the material would be a more difficult task to complete since many of the pieces are unique. Mistakes happen and I would think that this is the case. I am sure he regrets this and has learned a hard lesson.
Here is an interesting post to our sister site, the Coin Community Forum, which provides great details and follow-up colour of a smash and grab burglary of coins from a vehicle. I hope it is permissible for me to provide the link here.