Oh, I'd say that Doug the Thug's involvement in any of Slob's less-than-legal shenanigans is anything but accidental. Don't forget, Rob is genuinely a moron, while Douggie is known as being (well, comparitively) the more intelligent of the pair. If anything, I'd figure Doug's the one calling the shots. At the very least, they're partners in crime.
Of course, this is all speculation. Who knows, really?
If circumstances really do come to these two shitstains getting into serious trouble with the Law - including facing down a potentially lengthy stay at Her Majesty's pleasure in a dank prison cell featuring all the trimmings - I wonder if the ever-so vaunted Ford Family Loyalty will continue unabated...or will the boys each finally turn on one another in a feeble attempt to save their own worthless skins? The thought of Doug-o throwing his brother under the bus (and the subsequent look of betrayed astonishment on Slob's stupid fat face when it happens) is absolutely heart-warming, isn't it?