An In-App purchase on one iOS device can be used on multiple other iOS devices as long as the user signs in to the App Store with the same Apple ID. The app provides a “Restore Prior Purchase” button that can be used for this purpose. An In-App purchase on one OS X computer can be used on multiple other OS X computers, as well.
It is not possible, however, to apply the In-App purchase for an iOS device to an OS X computer, or vice versa. The App Store and Mac App Store are two distinct entities and an In-App purchase for an iOS device is a completely different product from an In-App purchase for an OS X computer.