[semlar]

Well, you can tell them that it isn't feasible to accurately detect the position of an enemy unit.

There have certainly been addons in the past that show how many friendly units are within range of an aoe heal, but what you're describing can't really be done.

You can get the position of units in your group, and through addon communication it is possible to transmit the coordinates of other units running the addon, but outside of that there are no api functions for getting real positional data.

There is one teeny tiny exception involving the combat log and friendly unit positions but as far as I know it's never actually been implemented into an addon and it wouldn't be reliable enough to use.