You took some precautions

The moment she finishes the incantation you drop your façade, smiling like a mad man, as you already knew her secret. The truth is that one day when you were still dating, you saw her cast a spell, turning her familiar into a grey tabby cat, and even though you didn't pay much mind to it when you were still with her, after you broke up you decided to take some precautions. Using the dark web and some simple spells of your own, you made your entire house and family spell proof, though, of course, it needs to go somewhere so you made it rebound back to the caster.

After she realizes what you did, she tries running away, but it's too late as her own spell is now changing her into something very special.