Is he arching his back or is he lifting his butt off the bench and kind of bridging? Arching your back helps put your put the weight where you have better leverages and decreases the distance the bar has to travel. When done correctly arch back benching is safer then how most people bench because it allows you to utilize your lats more to help and protects your shoulders. Some people definitely take it a little overboard but in sports like powerlifting everyone else is taking it overboard so why not be competitive with them.
As for being bad for your back, its not. First the weight is resting on your traps, not your spine therefore you increasing the stress to your spine. Secondly your lumbar spine has a natural arch to it, that you are not extending during an arch bench. The majority of your arch comes the thoracic area of your back. Finally as long as you are not neglecting training your posterior chain, your spinal erectors should be able to stabilize your arch so that you have a very solid base to push from, unlike a lot of flat benchers who kind of shift around all over the bench while pressing.