There are lots of different ways to do this. 
Since you’re familiar with core, my own suggestion would be that you set up a schedule for every other Saturday and simply disarm SHM based on the entry of her code and then re-arm it 15 minutes after she is scheduled to leave if you are also not home at that time.
There are a lot of other things that you could do to rearm it that would be more complicated, including using a virtual presence sensor or having her push a button or turn a particular light on and off, so just depends on how complicated you want it to be.
But the simplest approach is just to put it on a time schedule from the time that she arrives and just check that no one else is home then.
If you ask in the core peer Assistance thread, I’m sure there are people there who could help you set up something.