Find the Distance Between Two GPS Coordinate
Theoretically you would have to calculate the distance between two GPS Coordinate, Coordinate of the point and coordinate of the User , search for calculating distance between gps coordinates , you will get links to formulas to apply , once you get resulting distance you just have compare it to the needed range , you can do the math in B4A , i did that few years ago, i was working on my own GPS project , just search google for the phrase the distance or calculating the distance between two gps coordinates ,I Hope that helps, Good Luck