OK a thought as he is on maybe same router it will have same Ip address. But try this
Open your account ie here
You will see a subscription so click on the word subscription 1 and you will see a list of PCs protected. Each will have a name and nick name is your former PC there or do you see a name you did not have? If so click it and view in IP address column and you will see the IP address . I am unsure if this changes in real time but worth looking at and comparing to your Ip . If different this could be the IP the PC is on. If same it could be saying it is on your router.
Of course this is dependent on whether the IPs update in real time. Not exactly what you need but a start
It does not show if the PC is active. You can deactivate the licence on that PC as well.