Simon knew better than anyone what happened when a shifter became a witch's familiar. The witch's magic was heightened far more than it would have been with a regular animal but the same could be said for the shifter.
They had a magical bond. It was how he was able to find Delia every time she was reborn. All he had to do was learn how to tap into the magic tying their souls together.
She had taught him the first time she got her memories back in her second life. She made him promise that he would find her and break the curse. He made that promise so long ago and still hadn't managed to succeed but he might finally have a lead because of Maisie.
Simon and Delia had tried to break the curse together, of course. Every time she got her memories back, her magic was unsealed along with it. They always tried…then failed…then tried to focus on spending what little time they had left together.