It depends on how Apple implements it, I guess? Some Android skins show you a big red warning, disable the "install" button for a few seconds, and the cancel b

It depends on how Apple implements it, I guess? Some Android skins show you a big red warning, disable the "install" button for a few seconds, and the cancel button is the one with the bright colour. This, plus not allowing sideloading by default, seems to work well. A simple popup probably wouldn't be as effective.