Yes it will depreciate but then all new cars will, and it is something that we must accept if we want a new motor.
However.... Should he have brought a 2k pd diesel. IMO no. Tax, mpg, insurance grouping, will all be excessive in comparison to the new polo. Keep it long enough and it will outstrip the pd car in savings many times, resale will be higher as well due to the "greener" credentials it holds.