Since nobody asked the obvious???
Did they kill the lawn last fall? Did the lawn have crabgrass last year? If so leave it alone and let the pre do its thing and seed in fall. If the lawn had no crab before, the be up front and honest with customer and let them now you have zero idea if the seed you put down "now' will germinate but you are going to try a few techniques to do your best.
First use a high quality seed - Pennington Smart Seed with mycorrhizae, power seed it in deep and top dress it with finished compost. Water 3 to 4 times a day for 3 weeks - even if it just rained and plan on repeating process in fall - sorry great lawns cost $$$ - thats why we do what we do.