Volkswagen Golf mystery brake failure

We bought our used VW Golf in January and have been thrilled with it.

Yesterday, some friends borrowed it while we were away, and as they were driving it back into the garage the brakes failed and it rolled downhill into the wall, smashing the garage door and causing quite extensive damage to the front of the vehicle. Our friend had been pumping the brakes but they were totally unresponsive. Luckily they were not on the open road.

It was towed back to the garage we bought it from, and when they looked at it they said there was nothing wrong with the brakes. Has anyone heard of this happening before? Or could anyone give a likely reason? The car is looking like a write off and our insurance does not cover damage to our car so I would love there to be some explanation somewhere. I have not found much on forums around apart from possibly problems with brake fluid, but, the car had a full service when we bought it in January so I can't imagine it would be that.

Thank you very much.