How accurate is Find My App to locate friends?

It's the same accuracy as the GPS accuracy of the device. If the GPS on the iPhone can't obtain a good signal, it may use Wi-Fi triangulation which will reduce the accuracy.

GPS accuracy may be decreased depending on the phone's environment (i.e. a tunnel would not have great GPS, but standing in an open field would).

it request the location of the device so if the device has gps then gps accuracies

“How accurate”? Are you seeking a numeric answer?

It’s accurate.