“Find your IPhone” is built in FYI. All you got to do is share your location if both of you have the iPhones. It’s definitely an invasion of privacy. What reason did she give for wanting to have it ? If you said no, could she just install it behind your back? That’s straight up trust issues and route to divorce and hobby money spent on lawyers instead.
Just say yes. Then All you have to do is have a good excuse to be where you are when mongering. (As you should in case of tickets/acccidents/medical emergency, etc etc) You can flip the blame to her later on if she gets too crazy asking about your every move.
Then you can always turn off your phone or put it on airplane mode with wifi/Bluetooth off as well. To tell you from experience, these trackers aren’t that reliable. Especially if you have a cheaper phone plan or cheaper phone. Just say, no reception/phone froze/dead battery, etc etc.
Bottom line, what’s more important?