Help a couple that's been victims of theft?

Welcome to the Apple Community.

You can only locate your device when it is logged into iCloud and 'Find My Phone' is enabled, additionally the device will need to be switched on and connected to a wifi or cellular network.

Unfortunately, you cannot activate iCloud or 'Find My Phone' remotely.

If the device is wiped by you or another, you may not be able to locate the device (although some have reported they can)

The best place to try to locate the device is by logging into iCloud.comon a computer.

Yes, you can use the "Find my iPhone" app to locate each others devices assuming each device has the Find my iPhone service setup on it. That is exactly what the app is for, so just log in to the app using the AppleID and password of the device you want to locate.

Note that Find my iPhone is not intended for locating purposefully stolen devices (where the thief is likely to take every step possible to prevent you tracking the phone), it is primarily for locating lost/misplaced devices.