I suppose its cheap enough to try spraying again that soon but it would be best if he waited for whatever is there to recover I would think. Can you do the re-spray and seeding the following weekend? You may not even need to re-spray but I'm sure something will have survived if it was 3ft tall, either fescue not completely dead or canopy effect on weeds underneath.
What are you planting?
What are you planting?