Hi Sean, Kimberly here from lanscape-solutions-for-you.
Thank you for asking the question on your grass situation. Do you have any idea what type of chemical he put down on your grass? The most common is a grass killer, which will get the roots.
What I would suggest at this point, is to till the area over to get the dead grass out. Prep the area by raking and leveling out. You can add some top soil to the ground as option, depending on your soil conditions. Apply your seed, water, and put down straw to hold the moisture in. Get on a good watering schedule. The one caution is you live in a climate that will be, or is hot already. I would do this soon before the heat sets in. The key is preparing properly and watering.
I hope this has helped some. If you don't think the old grass root system is completely dead yet, you can also apply a grass killer to it, wait a few days and then till over and remove dead grass.
The best of luck to you and thank you again for asking the question here at landscape solutions for you. If you need further help or have other questions, please feel to ask right here. Have a great day!
Kimberly