"It'll be easier to take this thing down with him than without him."

And they do need something of his, after all, to take out the monster without him. Someone is going to have to go in that house to get it - better the person invited in than trying to break in and steal later. And she's also banking on Damien not knowing much about her to begin with.

Or what he will find might make him think they're on the same side.

"I'm not part of the Eye. I'm an outsider, like him. I stand a better shot."