I don't think creeping red or chewings fescue is suited for temps over 90 degrees. Kenblue is below average in quality.
The new weed could be crab, but weather has been a bit too cool for crabgrass...which needs air temps over 80.
Possibly the weed you see is a winter annual. For instance like chickweed, veronica, henbit...or your local favorite.
The Weed b Gone label allows treatment only after the third mowing.
Talk to the homeowner again and explain that you have a better and safer weed control product (Speedzone--four ingredients), and your power equipment would give more effective results. Blanket treatment. If he spot sprays --there is a risk of damaging the new grass--if he goes over the correct rate--or not killing the weeds if he goes less than the correct rate. Explain that you would include a fertilizer to help the new grass get thicker faster, and formulated to prepare it for the winter. Mention that he should throw his shoes away after he does the weed treatment--because unless he wears rubber boots--the chemical in his shoes will be absorbed through the skin of his feet for the next few months.