Yes, your friend would lose his deposit on the keg. There's always a deposit on kegs.
Not to mention the fact that you would have stolen a piece of equipment from a company worth upwards of a hundred dollars. I'm not one to stand up for BMC's business practices, but I imagine you wouldn't dream of walking into your local micro's tasting room and taking a hundred dollars out of the register. If that somehow feels different to you than keeping a keg that was loaned to you, you should spend some time thinking about the difficulty smaller breweries have getting kegs as it is. Losing one in the distribution pipeline is not only a big expense for the cost of the keg, the time and effort they expend looking for and replacing that will inevitably be just as big and inconvenient.
