Why don't you use Homekit for presence. One caveat is that your need a "recent" apple tv or ipad.
You can install a homebridge server on your local network and associate his phone with Homekit. You can then turn on a virtual switch in Hubitat when he arrives and turn it off when he leaves.
The phone can be on any network or cellular, doesn't have to be on the same has the homebridge server.
Take a look here:
and here:
I am with @Ryan780, I think that there is a way to make it a bit less complicated
