Changing tyres on RWD

A mate of mine has an RX8 (230BHP model) and recently got a puncture which meant the tyre(rear right) needed replacing. He got a top of the range Conti and the lad in the garage said his other rear tear was still in good nick and he did'nt need to replace it. About 1 week after the new tyre was on his got a chance put down the foot a bit and he said that the car was noticably losing traction on the side with the older tyre.. .. and after the long story , to the question, on a powerful RWD car should you always replace both tyres at the same time ?

always replace both driving wheel tyres at the same time!!!!!
that way you will be sure they will always respond in the same fashion to the same surface condition