ON AN IPHONE 5S, HOW DO I GET AN APP FROM MY IPHONE TO MY WIFE'S IPHONE?

Your wife should download / install it on her iPhone from AppStore.

Keep in mind that apps are permanently tied to the Apple ID that was used to download them.

Regards

Giulio

If its a paid App, its best to use Family Sharing.

Set up Family Sharing - Apple Support

If its a free App, just have her download it from the App Store. You can share a link to the App, by clicking the blue circle with the 3 white dots (...) in the Apps app store entry and then on Share App. You can use Airdrop or Email to share the link.

If the app you want is no longer available in the App Store then you're wife won't be able to use it.

Are you talking about this one?---> https://itunes.apple.com/us/app/find-my-car-car-locator/id1295565906?mt=8

Or about something different?

Giulio