I would have had the same concern, as you, but I probably wouldn't put my child in the car with some that had drank 4 beers, of course you also have to consider over how many hours did he drink. If he had been there for the whole day 4 beers wouldn't be bad but he had drank in a short period of time I would have been concerned. Your mother should have drove and probably wishes she had but won't admit it.