This was extremely fortunate.
Thomas had always been devoted to family. Of course, being part of that family had prevented her from using that to its full potential, limiting her to just Justine as leverage.
But Harry Dresden….
It was a dream come true - blackmail on the loose cannon wizard and additional leverage on Thomas in one fell swoop. And Dresden had proved to be more clever than anyone had given him credit for (genuinely surprising her, which wouldn't happen again), which could prove advantageous as long as she kept him as an ally.
Yes.
This was extremely fortunate.