if all your good players are gone you have basically started over, you should have already added to your arena so you are in better shape starting from here than if you just started over. There is no reason you cant go from where you are now manage your team well and be in Div 3 or competing for champioships in div 4 in no more than 3 seasons.
D.V to D.III is indeed possible to achieve in three seasons. But he is going to have a plan, and learn how to execute it first. Also a bit of luck as there are a few IV "sinkholes" that if promoted into could make your stay there several seasons. But the jump to IV should be pretty quick with good management.