First thing would be to determine if plain old iOS presence is reliable for her.
I recommend getting a Meross HomeKit compatible smart plug or power strip with one independently controlled socket for each member of your household that you want to track.
(You didn’t say what country you are in, but this is a relatively inexpensive device available in both the
Us And UK. And it can work with both HomeKit and ST simultaneously.)
They have 1, 2, 3, and 4 socket models. Make sure it says HomeKit compatible: they have a slightly cheaper line that isn’t, but you need HoneKit for this project.
Now create a HomeKit automation so that when your wife arrives home, one socket turns on, and when she leaves, it turns off.
Homekit Automation
If that works well, you’re on your way. 
Since both HomeKit and SmartThings can recognize the Meross at the same time, you can just use the socket as a proxy for presence: if the socket is on, your wife is home.
SmartThings routine
If you want to get fancier, you can create a virtual presence sensor and turn it on when the socket is on.
[ST Edge] vEdge Creator: a virtual device generator for end users
That’s your choice, but for most people just the socket should be enough.
All of this is just a method for letting you use the phone’s presence in ST without requiring the ST app to track presence since it doesn’t seem to be very good at it for some people.
If HomeKit presence isn’t reliable either, then that’s a different issue and would require a different solution.