I have two suggestions:
a) Can you give Sarah access to your WiFi? And then use something like @jwetzel1492's iPhone WiFi presence sensor (works with iPhone & Android) to determine when she's left?
b) I have a Fingbox (cost all of $99 a couple years ago). It integrates with IFTTT, and I trigger a Hubitat switch to turn on/off when the Fingbox detects/no-longer-detects Pattie's phone. Using a Fingbox obviates the need to provide access to your WiFi network.
And now, I must go back to watching football 
