It sounds a bit basic, but Bin-die works way better with the watering can method - especially if the ground is dry. Dunno why - but it does.
Follow the application rate per square metres especially on buffalo. If there is clover, give the clover an application first before doing the entire lawn (so the clover gets a double dose)
For your customer, the bindis will seem to be getting worse as they die and the seed balls dry out. It means you've sprayed the bindies a bit late.
For Illawarra NSW I spray late September and mid December. This hits the spring growth and the late stragglers. Do this 3 years in a row and you'll have eliminated 95% of the bindies.
Best way to completely rid bindies from the lawn is to make the lawn healthy. Light fertilise spring and autumn, keep at least 2 inches of depth in the grass. Once the bindies are gone mulch mow every so often.
Best way to grow a mighty crop of bindies is to mulch mow nice and short every 2 weeks year round - like a lot of grassies do.