Fuel leak XR 650 L Ascerbis Tank

Hi, A mate of mine bought a second hand 650l today from a dealer in Knysna, it was delivered by the salesman to him at his holiday spot in Sedgefield. All was good until my friend took it for a spin around the block soon after delivery and when he pulled up back at the house fuel started pouring out from under the tank somewhere. It was getting dark and the engine was hot so we had a cautious half look to try see where the fuel was coming from but couldn't see the exact source. The tank has 2 fuel cocks and we closed both of of these and fuel leak stopped. As soon as we opened either 1 again, the leak started to flow.
Any ideas or anyone had similar experience with this? Thanks.