If he loaned you the car it should have ins. that covers you. You should be liable for the deductible and nothing more. And if you have a gold or platinum card, that would cover it, Been there done that.
If it's in your possession, it's your responsibility. Even if the garage owner has full insurance on everything, maybe he doesn't want to file a claim? Why should he get a rate hike, or lose his coverage because he loans out his cars?