What kind of grass, and which product? If it is warm season grass like Bermuda, Saint Augustine, or Zoysia and the product had pre-emergence no harm done other than waisted money paying for pre-emergence. Other than that the nitrogen will just feed the grass like any other product.

IF it is a cool season grass is when you run into problems like force it to grow when it should be dormant or possible burn the grass. The other problem is if it has pre-emergence in it, and he/she plans to overseed this fall just flushed that idea down the tubes or at least severly hampered the plan.