Found this. Let us know if it will work in your case. I copied and pasted it so the reference is not to your wife, its someone else's.
Only one of your devices that share an iCloud account can be monitored by somebody else (i.e. your wife) using a different iCloud account. If you go into the 'Find My Friends' app on one of your devices, and click on 'Account', you will see a section titled 'Active Device'. That is where you identify which device you want to be tracked (it shows all devices using the same iCloud account).
The only way I could see to get around this is to create unique iCloud accounts for each of your devices and have your wife track all of them - then you wouldn't need to change which device is active as you yourself change which device you are carrying at any given time. Those accounts would only be used for tracking - your primary iCloud account would still be on each device for the purposes of syncing.
Credit: Apple Support Communities.
Sent from my iPhone using iPF