Large areas have turned brown. According to the homeowner, this happened very quickly.
Pythium is a disease that can be highly contagious and does spread quickly. Often, the lawn mower can be a caouse of spreading the disease. In this situation, you can see the marks left by the mower wheels in the lawn. Where the disease was spread by the blades and mower body the pattern is less distinct.
Some cases the lawn deteriorates quickly. Most often, the lawn will recover, but it may take some time. Overseeding will help to fill in some of the damaged areas.
To prevent further spread, refrain from watering that will leave the lawn damp through the night hours. Do not mow when the lawn is wet or in hot temperatures. Mow at 3" high minimum. Use a slow release fertilizer. We'd recommend something with 75% slow release nitrogen. Finally, mow the diseased areas last to minimize spread to the healthy parts.