No, use her

Matt left her in the bathroom and began to read the book again. She couldn't be trusted, but that didn't mean she couldn't be used. He could use her for a summoning material surely. Summon a powerful creature and get rid of her all in one stroke. That or control her somehow so he could trust her.
Searching the book, toward the back he found an old piece of parchment with something scrawled in archaic English. Matt smiled, this would be perfect.