Peugeot 107 won't start

I gave my son my car as I bought a new one. He is a young driver and was asked in attach a monitor to the car by way of the 12V connection on the dashboard. When he did this the car wouldn't start at all. The dashboard lit up, but there was no power to the engine, he called the breakdown service and they started it using the jumpleads and said the battery was flat, he drove it around for about an hour returned to the driveway and again nothing. I am reluctan to purchase a new battery, as I feel the problem was with the installation of the insurance gadget, as the battery was completely fine minutes before. please advise what to do next.